Data transmission method in wireless communication system, and apparatus therefor

ABSTRACT

A Station (STA) in a wireless communication system, the STA including a transceiver configured to transmit and receive a wireless signal; and a processor configured to control the transceiver. Further, the processor is further configured to: receive, from An access point (AP), a downlink (DL) multi-user (MU) Physical Protocol Data Unit (PPDU), wherein the DL MU PPDU includes a DL data and trigger frame for an uplink (UL) orthogonal frequency-division multiple access (OFDMA) transmission; and transmit, to the AP, an UL MU PPDU generated based on the DL MU PPDU, wherein the trigger frame is transmitted in a first frequency region of the DL MU PPDU, and the DL data is transmitted in a second frequency region of the DL MU PPDU, when the trigger frame is for multiple stations (STAs), and wherein the first frequency region and the second frequency region are different frequency region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of co-pending U.S. patent applicationSer. No. 15/506,632 filed on Feb. 24, 2017, which is the National Phaseof PCT International Application No. PCT/KR2015/008993 filed on Aug. 27,2015, which claims the priority benefit under 35 U.S.C. § 119(e) to U.S.Provisional Application Nos. 62/100,065 filed on Jan. 6, 2015 and62/042,759 filed on Aug. 27, 2014, all of which are hereby expresslyincorporated by reference into the present application.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to wireless communication systems, andmore particularly, to a method for transmitting data for supporting adata transmission of multi-user and a device for supporting the same.

Discussion of the Related Art

Wi-Fi is a wireless local area network (WLAN) technology which enables adevice to access the Internet in a frequency band of 2.4 GHz, 5 GHz or60 GHz.

A WLAN is based on the institute of electrical and electronic engineers(IEEE) 802.11 standard. The wireless next generation standing committee(WNG SC) of IEEE 802.11 is an ad-hoc committee which is worried aboutthe next-generation wireless local area network (WLAN) in the medium tolonger term.

IEEE 802.11n has an object of increasing the speed and reliability of anetwork and extending the coverage of a wireless network. Morespecifically, IEEE 802.11n supports a high throughput (HT) providing amaximum data rate of 600 Mbps. Furthermore, in order to minimize atransfer error and to optimize a data rate, IEEE 802.11n is based on amultiple inputs and multiple outputs (MIMO) technology in which multipleantennas are used at both ends of a transmission unit and a receptionunit.

As the spread of a WLAN is activated and applications using the WLAN arediversified, in the next-generation WLAN system supporting a very highthroughput (VHT), IEEE 802.11ac has been newly enacted as the nextversion of an IEEE 802.11n WLAN system. IEEE 802.11ac supports a datarate of 1 Gbps or more through 80 MHz bandwidth transmission and/orhigher bandwidth transmission (e.g., 160 MHz), and chiefly operates in a5 GHz band.

Recently, a need for a new WLAN system for supporting a higherthroughput than a data rate supported by IEEE 802.11ac comes to thefore.

The scope of IEEE 802.11ax chiefly discussed in the next-generation WLANtask group called a so-called IEEE 802.11ax or high efficiency (HEW)WLAN includes 1) the improvement of an 802.11 physical (PHY) layer andmedium access control (MAC) layer in bands of 2.4 GHz, 5 GHz, etc., 2)the improvement of spectrum efficiency and area throughput, 3) theimprovement of performance in actual indoor and outdoor environments,such as an environment in which an interference source is present, adense heterogeneous network environment, and an environment in which ahigh user load is present and so on.

A scenario chiefly taken into consideration in IEEE 802.11ax is a denseenvironment in which many access points (APs) and many stations (STAs)are present. In IEEE 802.11ax, the improvement of spectrum efficiencyand area throughput is discussed in such a situation. More specifically,there is an interest in the improvement of substantial performance inoutdoor environments not greatly taken into consideration in existingWLANs in addition to indoor environments.

In IEEE 802.11ax, there is a great interest in scenarios, such aswireless offices, smart homes, stadiums, hotspots, andbuildings/apartments. The improvement of system performance in a denseenvironment in which many APs and many STAs are present is discussedbased on the corresponding scenarios.

In the future, it is expected in IEEE 802.11ax that the improvement ofsystem performance in an overlapping basic service set (OBSS)environment, the improvement of an outdoor environment, cellularoffloading, and so on rather than single link performance improvement ina single basic service set (BSS) will be actively discussed. Thedirectivity of such IEEE 802.11ax means that the next-generation WLANwill have a technical scope gradually similar to that of mobilecommunication. Recently, when considering a situation in which mobilecommunication and a WLAN technology are discussed together in smallcells and direct-to-direct (D2D) communication coverage, it is expectedthat the technological and business convergence of the next-generationWLAN based on IEEE 802.11ax and mobile communication will be furtheractivated.

SUMMARY OF THE INVENTION

An object of the present invention is to propose an uplink/downlinkmulti-user data transmission and reception method in a wirelesscommunication system.

In addition, an object of the present invention is to propose a HighEfficiency (HE) format of a PPDU used for an uplink/downlink multi-usertransmission and reception in a wireless communication system.

In addition, an object of the present invention is to propose an HEformat of a DL/UL MU PPDU of a cascade scheme for efficiently utilizingDL/UL MU resource.

The technical objects of the present invention are not limited to thoseobjects described above; other technical objects not mentioned above maybe clearly understood from what are described below by those skilled inthe art to which the present invention belongs.

In order to solve the technical problem, according to an embodiment ofthe present invention, an AP device of a WLAN system and a method fortransmitting data performed by the AP device are proposed.

According to an aspect of the present invention, a method for performinga downlink (DL) multi-user (MU) transmission by an access point (AP) ina wireless communication system may include transmitting a DL MUPhysical Protocol Data Unit (PPDU), where the DL MU PPDU includes atrigger frame including trigger information for an uplink (UL) MUtransmission and a DL MU frame; and receiving a UL MU PPDU generatedbased on the DL MU PPDU through UL MU, where the UL MU PPDU includes aUL MU frame based on the trigger frame and an acknowledge (ACK) frame inresponse to the DL MU frame.

In addition, when the trigger frame is transmitted together to areceiving station (STA) that receives the DL MU frame, the DL MU frameand the trigger frame may be included as an Aggregated MAC Protocol DataUnit (A-MPDU) format in a data field allocated to the receiving STA.

In addition, a MAC header of the trigger frame may include a triggerindicator indicating that the trigger frame includes the triggerinformation.

In addition, the trigger frame may correspond to a first MPDU among MACProtocol Data Units (MPDUs) included in the A-MPDU format.

In addition, when the trigger frame is not transmitted together to areceiving station (STA) that receives the DL MU frame, the trigger framemay be included in a preconfigured data field of the DL MU PPDU as anMPDU format, and the DL MU frame may be included in other data field inthe DL MU PPDU as the MPDU format or an A-MPDU format.

In addition, a physical frame in the DL MU PPDU may include a triggerindicator indicating whether the DL MU PPDU includes the trigger frame,or Broadcast Association Identifier (AID) information that receives thepreconfigured data field.

In addition, the trigger indicator or the broadcast AID information maybe included in a High Efficiency-Signal (HE-SIG) field of the physicalpreamble.

In addition, when the trigger frame is transmitted together to areceiving station (STA) that receives the DL MU frame, the ACK frame maybe included in a preconfigured data field of the UL MU PPDU as an MPDUformat, and the UL MU frame is included in other data field as the MPDUformat or an A-MPDU format.

In addition, when the trigger frame is transmitted together to areceiving station (STA) that receives the DL MU frame, the ACK frame maybe included in a preconfigured data field in the UL MU PPDU by beingpiggybacked in the UL MU frame.

In addition, the trigger frame may include at least one of AssociationIdentifier (AID) information of an STA that performs the UL MUtransmission, space resource indication information for the UL MUtransmission and frequency resource indication information for the UL MUtransmission.

In addition, according to another aspect of the present invention, anaccess point (AP) device in a wireless communication system may includean RF unit for transmitting and receiving a wireless signal; and aprocessor for controlling the RF unit, wherein the processor isconfigured to perform: transmitting a DL MU Physical Protocol Data Unit(PPDU), where the DL MU PPDU includes a trigger frame including triggerinformation for an uplink (UL) MU transmission and a DL MU frame; andreceiving a UL MU PPDU generated based on the DL MU PPDU through UL MU,where the UL MU PPDU includes a UL MU frame based on the trigger frameand an acknowledge (ACK) frame in response to the DL MU frame.

In addition, when the trigger frame is transmitted together to areceiving station (STA) that receives the DL MU frame, the DL MU frameand the trigger frame may be included as an Aggregated MAC Protocol DataUnit (A-MPDU) format in a data field allocated to the receiving STA.

In addition, when the trigger frame is not transmitted together to areceiving station (STA) that receives the DL MU frame, the trigger framemay be included in a preconfigured data field of the DL MU PPDU as anMPDU format, and the DL MU frame may be included in other data field inthe DL MU PPDU as the MPDU format or an A-MPDU format.

According to an embodiment of the present invention, a trigger frame anda DL MU frame are transmitted simultaneously through a single DL MUPPDU, and accordingly, there is an effect that the DL MU resource issaved and the communication performance is improved.

In addition, according to an embodiment of the present invention, an ACKframe and a UL MU frame are transmitted simultaneously through a singleUL MU PPDU, and accordingly, there is an effect that the UL MU resourceis saved and the communication performance is improved.

Other effects of the present invention are additionally discussed in theembodiments below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention. In the drawings:

FIG. 1 is a diagram showing an example of an IEEE 802.11 system to whichthe present invention may be applied;

FIG. 2 is a diagram illustrating the structure of a layer architectureof an IEEE 802.11 system to which the present invention may be applied;

FIG. 3 illustrates a non-HT format PPDU and an HT format PPDU in awireless communication system to which the present invention may beapplied;

FIG. 4 illustrates a VHT format PPDU in a wireless communication systemto which the present invention may be applied;

FIG. 5 illustrates constellation diagrams for classifying a PPDU formatin a wireless communication system to which the present invention may beapplied;

FIG. 6 illustrates a MAC frame format in an IEEE 802.11 system to whichthe present invention may be applied;

FIG. 7 is a diagram illustrating the frame control field in the MACframe in a wireless communication system to which the present inventionmay be applied;

FIG. 8 illustrates the VHT format of an HT control field in a wirelesscommunication system to which the present invention may be applied;

FIG. 9 is a diagram illustrating a random backoff period and a frametransmission procedure in a wireless communication system to which thepresent invention may be applied;

FIG. 10 is a diagram illustrating an IFS relation in a wirelesscommunication system to which the present invention may be applied;

FIG. 11 is a diagram conceptually showing a method of channel soundingin a wireless communication system to which the present invention may beapplied;

FIG. 12 is a diagram illustrating a VHT NDPA frame in a wirelesscommunication system to which the present invention may be applied;

FIG. 13 is a diagram illustrating an NDP PPDU in a wirelesscommunication system to which the present invention may be applied;

FIG. 14 is a diagram illustrating a VHT compressed beamforming frameformat in a wireless communication system to which the present inventionmay be applied;

FIG. 15 is a diagram illustrating a Beamforming Report Poll frame formatin a wireless communication system to which the present invention may beapplied;

FIG. 16 is a diagram illustrating a Group ID Management frame in awireless communication system to which the present invention may beapplied;

FIG. 17 is a diagram illustrating a downlink multi-user PPDU format in awireless communication system to which the present invention may beapplied;

FIG. 18 is a diagram illustrating a downlink multi-user PPDU format in awireless communication system to which the present invention may beapplied;

FIG. 19 is a diagram illustrating a downlink MU-MIMO transmissionprocess in a wireless communication system to which the presentinvention may be applied;

FIG. 20 is a diagram illustrating an ACK frame in a wirelesscommunication system to which the present invention may be applied;

FIG. 21 is a diagram illustrating a Block Ack Request frame in awireless communication system to which the present invention may beapplied;

FIG. 22 is a diagram illustrating the BAR Information field of a BlockAck Request frame in a wireless communication system to which thepresent invention may be applied;

FIG. 23 is a diagram illustrating a Block Ack frame in a wirelesscommunication system to which the present invention may be applied;

FIG. 24 is a diagram illustrating the BA Information field of a BlockAck frame in a wireless communication system to which the presentinvention may be applied;

FIG. 25 is a diagram illustrating a high efficiency (HE) format PPDUaccording to an embodiment of the present invention;

FIGS. 26 to 29 are diagrams illustrating HE format PPDUs according to anembodiment of the present invention;

FIG. 30 illustrates an example of phase rotation for the classificationof HE format PPDUs;

FIG. 31 is a diagram illustrating an uplink multi-user transmissionprocedure according to an embodiment of the present invention;

FIG. 32 is a diagram illustrating an uplink multi-user transmissionaccording to an embodiment of the present invention;

FIG. 33 is a diagram illustrating resource allocation units in an OFDMmulti-user transmission scheme according to an embodiment of the presentinvention;

FIG. 34 is a diagram illustrating a multi-user transmission procedureaccording to an embodiment of the present invention;

FIG. 35 is a diagram exemplifying a PPDU structure that carries triggerinformation according to an embodiment of the present invention;

FIG. 36 is a diagram illustrating a DL MU PPDU structure according to afirst embodiment of the present invention;

FIG. 37 is a diagram illustrating a DL MU PPDU and a UL MU PPDUtransmitted in response to the DL MU PPDU according to an embodiment ofthe present invention;

FIG. 38 is a diagram illustrating a DL MU PPDU and a UL MU PPDUaccording to another embodiment of the present invention;

FIG. 39 is a diagram illustrating a DL MU PPDU structure according tothe embodiment 2-1 of the present invention;

FIG. 40 is a diagram illustrating a DL MU PPDU structure according tothe embodiment 2-2 of the present invention;

FIG. 41 illustrates a table comparing the first and second embodiments;

FIG. 42 is a diagram illustrating a UL MU PPDU according to anembodiment of the present invention;

FIG. 43 is a flowchart illustrating a DL MU transmission methodperformed by an AP according to an embodiment of the present invention;and

FIG. 44 is a block diagram of each STA device according to an embodimentof the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the exemplary embodiments of thepresent invention with reference to the accompanying drawings. Thedetailed description, which will be given below with reference to theaccompanying drawings, is intended to explain exemplary embodiments ofthe present invention, rather than to show the only embodiments that maybe implemented according to the invention. The following detaileddescription includes specific details in order to provide a thoroughunderstanding of the present invention. However, it will be apparent tothose skilled in the art that the present invention may be practicedwithout such specific details.

In some instances, well-known structures and devices are omitted inorder to avoid obscuring the concepts of the present invention andimportant functions of the structures and devices are shown in blockdiagram form.

It should be noted that specific terms disclosed in the presentinvention are proposed for convenience of description and betterunderstanding of the present invention, and the use of these specificterms may be changed to other formats within the technical scope orspirit of the present invention.

The following technologies may be used in a variety of wirelesscommunication systems, such as code division multiple access (CDMA),frequency division multiple access (FDMA), time division multiple access(TDMA), orthogonal frequency division multiple access (OFDMA), singlecarrier frequency division multiple access (SC-FDMA), and non-orthogonalmultiple access (NOMA). CDMA may be implemented using a radiotechnology, such as universal terrestrial radio access (UTRA) orCDMA2000. TDMA may be implemented using a radio technology, such asglobal system for Mobile communications (GSM)/general packet radioservice (GPRS)/enhanced data rates for GSM evolution (EDGE). OFDMA maybe implemented using a radio technology, such as institute of electricaland electronics engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16 (WiMAX),IEEE 802.20, or evolved UTRA (E-UTRA). UTRA is part of a universalmobile telecommunications system (UMTS). 3rd generation partnershipproject (3GPP) long term evolution (LTE) is part of an evolved UMTS(E-UMTS) using evolved UMTS terrestrial radio access (E-UTRA), and itadopts OFDMA in downlink and adopts SC-FDMA in uplink. LTE-advanced(LTE-A) is the evolution of 3GPP LTE.

Embodiments of the present invention may be supported by the standarddocuments disclosed in at least one of IEEE 802, 3GPP, and 3GPP2, thatis, radio access systems. That is, steps or portions that belong to theembodiments of the present invention and that are not described in orderto clearly expose the technical spirit of the present invention may besupported by the documents. Furthermore, all terms disclosed in thisdocument may be described by the standard documents.

In order to more clarify a description, 3GPP LTE/LTE-A is chieflydescribed, but the technical characteristics of the present inventionare not limited thereto.

General System

FIG. 1 is a diagram showing an example of an IEEE 802.11 system to whichan embodiment of the present invention may be applied.

The IEEE 802.11 configuration may include a plurality of elements. Theremay be provided a wireless communication system supporting transparentstation (STA) mobility for a higher layer through an interaction betweenthe elements. A basic service set (BSS) may correspond to a basicconfiguration block in an IEEE 802.11 system.

FIG. 1 illustrates that three BSSs BSS 1 to BSS 3 are present and twoSTAs (e.g., an STA 1 and an STA 2 are included in the BSS 1, an STA 3and an STA 4 are included in the BSS 2, and an STA 5 and an STA 6 areincluded in the BSS 3) are included as the members of each BSS.

In FIG. 1, an ellipse indicative of a BSS may be interpreted as beingindicative of a coverage area in which STAs included in thecorresponding BSS maintain communication. Such an area may be called abasic service area (BSA). When an STA moves outside the BSA, it isunable to directly communicate with other STAs within the correspondingBSA.

In the IEEE 802.11 system, the most basic type of a BSS is anindependent a BSS (IBSS). For example, an IBSS may have a minimum formincluding only two STAs. Furthermore, the BSS 3 of FIG. 1 which is thesimplest form and from which other elements have been omitted maycorrespond to a representative example of the IBSS. Such a configurationmay be possible if STAs can directly communicate with each other.Furthermore, a LAN of such a form is not previously planned andconfigured, but may be configured when it is necessary. This may also becalled an ad-hoc network.

When an STA is powered off or on or an STA enters into or exits from aBSS area, the membership of the STA in the BSS may be dynamicallychanged. In order to become a member of a BSS, an STA may join the BSSusing a synchronization process. In order to access all of services in aBSS-based configuration, an STA needs to be associated with the BSS.Such association may be dynamically configured, and may include the useof a distribution system service (DSS).

In an 802.11 system, the distance of a direct STA-to-STA may beconstrained by physical layer (PHY) performance. In any case, the limitof such a distance may be sufficient, but communication between STAs ina longer distance may be required, if necessary. In order to supportextended coverage, a distribution system (DS) may be configured.

The DS means a configuration in which BSSs are interconnected. Morespecifically, a BSS may be present as an element of an extended form ofa network including a plurality of BSSs instead of an independent BSS asin FIG. 1.

The DS is a logical concept and may be specified by the characteristicsof a distribution system medium (DSM). In the IEEE 802.11 standard, awireless medium (WM) and a distribution system medium (DSM) arelogically divided. Each logical medium is used for a different purposeand used by a different element. In the definition of the IEEE 802.11standard, such media are not limited to the same one and are also notlimited to different ones. The flexibility of the configuration (i.e., aDS configuration or another network configuration) of an IEEE 802.11system may be described in that a plurality of media is logicallydifferent as described above. That is, an IEEE 802.11 systemconfiguration may be implemented in various ways, and a correspondingsystem configuration may be independently specified by the physicalcharacteristics of each implementation example.

The DS can support a mobile device by providing the seamless integrationof a plurality of BSSs and providing logical services required to handlean address to a destination.

An AP means an entity which enables access to a DS through a WM withrespect to associated STAs and has the STA functionality. The movementof data between a BSS and the DS can be performed through an AP. Forexample, each of the STA 2 and the STA 3 of FIG. 1 has the functionalityof an STA and provides a function which enables associated STAs (e.g.,the STA 1 and the STA 4) to access the DS. Furthermore, all of APsbasically correspond to an STA, and thus all of the APs are entitiescapable of being addressed. An address used by an AP for communicationon a WM and an address used by an AP for communication on a DSM may notneed to be necessarily the same.

Data transmitted from one of STAs, associated with an AP, to the STAaddress of the AP may be always received by an uncontrolled port andprocessed by an IEEE 802.1X port access entity. Furthermore, when acontrolled port is authenticated, transmission data (or frame) may bedelivered to a DS.

A wireless network having an arbitrary size and complexity may include aDS and BSSs. In an IEEE 802.11 system, a network of such a method iscalled an extended service set (ESS) network. The ESS may correspond toa set of BSSs connected to a single DS. However, the ESS does notinclude a DS. The ESS network is characterized in that it looks like anIBSS network in a logical link control (LLC) layer. STAs included in theESS may communicate with each other. Mobile STAs may move from one BSSto the other BSS (within the same ESS) in a manner transparent to theLLC layer.

In an IEEE 802.11 system, the relative physical positions of BSSs inFIG. 1 are not assumed, and the following forms are all possible.

More specifically, BSSs may partially overlap, which is a form commonlyused to provide consecutive coverage. Furthermore, BSSs may not bephysically connected, and logically there is no limit to the distancebetween BSSs. Furthermore, BSSs may be placed in the same positionphysically and may be used to provide redundancy. Furthermore, one (orone or more) IBSS or ESS networks may be physically present in the samespace as one or more ESS networks. This may correspond to an ESS networkform if an ad-hoc network operates at the position in which an ESSnetwork is present, if IEEE 802.11 networks that physically overlap areconfigured by different organizations, or if two or more differentaccess and security policies are required at the same position.

In a WLAN system, an STA is an apparatus operating in accordance withthe medium access control (MAC)/PHY regulations of IEEE 802.11. An STAmay include an AP STA and a non-AP STA unless the functionality of theSTA is not individually different from that of an AP. In this case,assuming that communication is performed between an STA and an AP, theSTA may be interpreted as being a non-AP STA. In the example of FIG. 1,the STA 1, the STA 4, the STA 5, and the STA 6 correspond to non-APSTAs, and the STA 2 and the STA 3 correspond to AP STAs.

A non-AP STA corresponds to an apparatus directly handled by a user,such as a laptop computer or a mobile phone. In the followingdescription, a non-AP STA may also be called a wireless device, aterminal, user equipment (UE), a mobile station (MS), a mobile terminal,a wireless terminal, a wireless transmit/receive unit (WTRU), a networkinterface device, a machine-type communication (MTC) device, amachine-to-machine (M2M) device or the like.

Furthermore, an AP is a concept corresponding to a base station (BS), anode-B, an evolved Node-B (eNB), a base transceiver system (BTS), afemto BS or the like in other wireless communication fields.

Hereinafter, in this specification, downlink (DL) means communicationfrom an AP to a non-AP STA. Uplink (UL) means communication from anon-AP STA to an AP. In DL, a transmitter may be part of an AP, and areceiver may be part of a non-AP STA. In UL, a transmitter may be partof a non-AP STA, and a receiver may be part of an AP.

FIG. 2 is a diagram illustrating the structure of a layer architectureof an IEEE 802.11 system to which an embodiment of the present inventionmay be applied.

Referring to FIG. 2, the layer architecture of the IEEE 802.11 systemmay include an MAC sublayer and a PHY sublayer.

The PHY sublayer may be divided into a physical layer convergenceprocedure (PLCP) entity and a physical medium dependent (PMD) entity. Inthis case, the PLCP entity functions to connect the MAC sublayer and adata frame, and the PMD entity functions to wirelessly transmit andreceive data to and from two or more STAs.

The MAC sublayer and the PHY sublayer may include respective managemententities, which may be referred to as an MAC sublayer management entity(MLME) and a PHY sublayer management entity (PLME), respectively. Themanagement entities provide a layer management service interface throughthe operation of a layer management function. The MLME is connected tothe PLME and may perform the management operation of the MAC sublayer.Likewise, the PLME is also connected to the MLME and may perform themanagement operation of the PHY sublayer.

In order to provide a precise MAC operation, a station management entity(SME) may be present in each STA. The SME is a management entityindependent of each layer, and collects layer-based state informationfrom the MLME and the PLME or sets the values of layer-specificparameters. The SME may perform such a function instead of common systemmanagement entities and may implement a standard management protocol.

The MLME, the PLME, and the SME may interact with each other usingvarious methods based on primitives. More specifically, anXX-GET.request primitive is used to request the value of a managementinformation base (MIB) attribute. An XX-GET.confirm primitive returnsthe value of a corresponding MIB attribute if the state is “SUCCESS”,and indicates an error in the state field and returns the value in othercases. An XX-SET.request primitive is used to make a request so that adesignated MIB attribute is set as a given value. If an MIB attributemeans a specific operation, such a request requests the execution of thespecific operation. Furthermore, an XX-SET.confirm primitive means thata designated MIB attribute has been set as a requested value if thestate is “SUCCESS.” In other cases, the XX-SET.confirm primitiveindicates that the state field is an error situation. If an MIBattribute means a specific operation, the primitive may confirm that acorresponding operation has been performed.

An operation in each sublayer is described in brief as follows.

The MAC sublayer generates one or more MAC protocol data units (MPDUs)by attaching an MAC header and a frame check sequence (FCS) to a MACservice data unit (MSDU) received from a higher layer (e.g., an LLClayer) or the fragment of the MSDU. The generated MPDU is delivered tothe PHY sublayer.

If an aggregated MSDU (A-MSDU) scheme is used, a plurality of MSDUs maybe aggregated into a single aggregated MSDU (A-MSDU). The MSDUaggregation operation may be performed in an MAC higher layer. TheA-MSDU is delivered to the PHY sublayer as a single MPDU (if it is notfragmented).

The PHY sublayer generates a physical protocol data unit (PPDU) byattaching an additional field, including information for a PHYtransceiver, to a physical service data unit (PSDU) received from theMAC sublayer. The PPDU is transmitted through a wireless medium.

The PSDU has been received by the PHY sublayer from the MAC sublayer,and the MPDU has been transmitted from the MAC sublayer to the PHYsublayer. Accordingly, the PSDU is substantially the same as the MPDU.

If an aggregated MPDU (A-MPDU) scheme is used, a plurality of MPDUs (inthis case, each MPDU may carry an A-MSDU) may be aggregated in a singleA-MPDU. The MPDU aggregation operation may be performed in an MAC lowerlayer. The A-MPDU may include an aggregation of various types of MPDUs(e.g., QoS data, acknowledge (ACK), and a block ACK (BlockAck)). The PHYsublayer receives an A-MPDU, that is, a single PSDU, from the MACsublayer. That is, the PSDU includes a plurality of MPDUs. Accordingly,the A-MPDU is transmitted through a wireless medium within a singlePPDU.

Physical Protocol Data Unit (PPDU) Format

A PPDU means a data block generated in the physical layer. A PPDU formatis described below based on an IEEE 802.11 a WLAN system to which anembodiment of the present invention may be applied.

FIG. 3 illustrates a non-HT format PPDU and an HT format PPDU in awireless communication system to which an embodiment of the presentinvention may be applied.

FIG. 3(a) illustrates a non-HT format PPDU for supporting IEEE 802.11a/gsystems. The non-HT PPDU may also be called a legacy PPDU.

Referring to FIG. 3(a), the non-HT format PPDU is configured to includea legacy format preamble, including a legacy (or non-HT) short trainingfield (L-STF), a legacy (or non-HT) long training field (L-LTF), and alegacy (or non-HT) signal (L-SIG) field, and a data field.

The L-STF may include a short training orthogonal frequency divisionmultiplexing symbol (OFDM). The L-STF may be used for frame timingacquisition, automatic gain control (AGC), diversity detection, andcoarse frequency/time synchronization.

The L-LTF may include a long training OFDM symbol. The L-LTF may be usedfor fine frequency/time synchronization and channel estimation.

The L-SIG field may be used to send control information for thedemodulation and decoding of the data field.

The L-SIG field may include a rate field of four bits, a reserved fieldof 1 bit, a length field of 12 bits, a parity bit of 1 bit, and a signaltail field of 6 bits.

The rate field includes transfer rate information, and the length fieldindicates the number of octets of a PSDU.

FIG. 3(b) illustrates an HT mixed format PPDU for supporting both anIEEE 802.11n system and IEEE 802.11a/g system.

Referring to FIG. 3(b), the HT mixed format PPDU is configured toinclude a legacy format preamble including an L-STF, an L-LTF, and anL-SIG field, an HT format preamble including an HT-signal (HT-SIG)field, a HT short training field (HT-STF), and a HT long training field(HT-LTF), and a data field.

The L-STF, the L-LTF, and the L-SIG field mean legacy fields forbackward compatibility and are the same as those of the non-HT formatfrom the L-STF to the L-SIG field. An L-STA may interpret a data fieldthrough an L-LTF, an L-LTF, and an L-SIG field although it receives anHT mixed PPDU. In this case, the L-LTF may further include informationfor channel estimation to be performed by an HT-STA in order to receivethe HT mixed PPDU and to demodulate the L-SIG field and the HT-SIGfield.

An HT-STA may be aware of an HT mixed format PPDU using the HT-SIG fieldsubsequent to the legacy fields, and may decode the data field based onthe HT mixed format PPDU.

The HT-LTF may be used for channel estimation for the demodulation ofthe data field. IEEE 802.11n supports single user multi-input andmulti-output (SU-MIMO) and thus may include a plurality of HT-LTFs forchannel estimation with respect to each of data fields transmitted in aplurality of spatial streams.

The HT-LTF may include a data HT-LTF used for channel estimation for aspatial stream and an extension HT-LTF additionally used for fullchannel sounding. Accordingly, a plurality of HT-LTFs may be the same asor greater than the number of transmitted spatial streams.

In the HT mixed format PPDU, the L-STF, the L-LTF, and the L-SIG fieldsare first transmitted so that an L-STA can receive the L-STF, the L-LTF,and the L-SIG fields and obtain data. Thereafter, the HT-SIG field istransmitted for the demodulation and decoding of data transmitted for anHT-STA.

An L-STF, an L-LTF, L-SIG, and HT-SIG fields are transmitted withoutperforming beamforming up to an HT-SIG field so that an L-STA and anHT-STA can receive a corresponding PPDU and obtain data. In an HT-STF,an HT-LTF, and a data field that are subsequently transmitted, radiosignals are transmitted through precoding. In this case, an HT-STF istransmitted so that an STA receiving a corresponding PPDU by performingprecoding may take into considerate a portion whose power is varied byprecoding, and a plurality of HT-LTFs and a data field are subsequentlytransmitted.

Table 1 below illustrates the HT-SIG field.

TABLE 1 Field Bit Description MCS 7 Indicate a modulation and codingscheme CBW 20/40 1 Set to “0” if a CBW is 20 MHz or 40 MHz orupper/lower Set to “1” if a CBW is 40 MHz HT length 16 Indicate thenumber of data octets within a PSDU Smoothing 1 Set to “1” if channelsmoothing is recommended Set to “0” if channel estimation is recommendedunsmoothingly for each carrier Not-sounding 1 Set to “0” if a PPDU is asounding PPDU Set to “1” if a PPDU is not a sounding PPDU Reserved 1 Setto “1” Aggregation 1 Set to “1” if a PPDU includes an A-MPDU Set to “0”if not Space-time 2 Indicate a difference between the number ofspace-time streams block coding (NSTS) and the number of spatial streams(NSS) indicated by an (STBC) MCS Set to “00” if an STBC is not used FECcoding 1 Set to “1” if low-density parity check (LDPC) is used Set to“0” if binary convolutional code (BCC) is used Short GI 1 Set to “1” ifa short guard interval (GI) is used after HT training Set to “0” if notNumber of 2 Indicate the number of extension spatial streams (NESSs)extension Set to “0” if there is no NESS spatial streams Set to “1” ifthe number of NESSs is 1 Set to “2” if the number of NESSs is 2 Set to“3” if the number of NESSs is 3 CRC 8 Include CRS for detecting an errorof a PPDU on the receiver side Tail bits 6 Used to terminate the trellisof a convolutional decoder Set to “0”

FIG. 3(c) illustrates an HT-green field format PPDU (HT-GF format PPDU)for supporting only an IEEE 802.11n system.

Referring to FIG. 3(c), the HT-GF format PPDU includes an HT-GF-STF, anHT-LTF1, an HT-SIG field, a plurality of HT-LTF2s, and a data field.

The HT-GF-STF is used for frame timing acquisition and AGC.

The HT-LTF1 is used for channel estimation.

The HT-SIG field is used for the demodulation and decoding of the datafield.

The HT-LTF2 is used for channel estimation for the demodulation of thedata field. Likewise, an HT-STA uses SU-MIMO. Accordingly, a pluralityof the HT-LTF2s may be configured because channel estimation isnecessary for each of data fields transmitted in a plurality of spatialstreams.

The plurality of HT-LTF2s may include a plurality of data HT-LTFs and aplurality of extension HT-LTFs like the HT-LTF of the HT mixed PPDU.

In FIGS. 3(a) to 3(c), the data field is a payload and may include aservice field, a scrambled PSDU (PSDU) field, tail bits, and paddingbits. All of the bits of the data field are scrambled.

FIG. 3(d) illustrates a service field included in the data field. Theservice field has 16 bits. The 16 bits are assigned No. 0 to No. 15 andare sequentially transmitted from the No. 0 bit. The No. 0 bit to theNo. 6 bit are set to 0 and are used to synchronize a descrambler withina reception stage.

An IEEE 802.11ac WLAN system supports the transmission of a DLmulti-user multiple input multiple output (MU-MIMO) method in which aplurality of STAs accesses a channel at the same time in order toefficiently use a radio channel. In accordance with the MU-MIMOtransmission method, an AP may simultaneously transmit a packet to oneor more STAs that have been subjected to MIMO pairing.

Downlink multi-user transmission (DL MU transmission) means a technologyin which an AP transmits a PPDU to a plurality of non-AP STAs throughthe same time resources using one or more antennas.

Hereinafter, an MU PPDU means a PPDU which delivers one or more PSDUsfor one or more STAs using the MU-MIMO technology or the OFDMAtechnology. Furthermore, an SU PPDU means a PPDU having a format inwhich only one PSDU can be delivered or which does not have a PSDU.

For MU-MIMO transmission, the size of control information transmitted toan STA may be relatively larger than the size of 802.11n controlinformation. Control information additionally required to supportMU-MIMO may include information indicating the number of spatial streamsreceived by each STA and information related to the modulation andcoding of data transmitted to each STA may correspond to the controlinformation, for example.

Accordingly, when MU-MIMO transmission is performed to provide aplurality of STAs with a data service at the same time, the size oftransmitted control information may be increased according to the numberof STAs which receive the control information.

In order to efficiently transmit the control information whose size isincreased as described above, a plurality of pieces of controlinformation required for MU-MIMO transmission may be divided into twotypes of control information: common control information that isrequired for all of STAs in common and dedicated control informationindividually required for a specific STA, and may be transmitted.

FIG. 4 illustrates a VHT format PPDU in a wireless communication systemto which an embodiment of the present invention may be applied.

FIG. 4(a) illustrates a VHT format PPDU for supporting an IEEE 802.11acsystem.

Referring to FIG. 4(a), the VHT format PPDU is configured to include alegacy format preamble including an L-STF, an L-LTF, and an L-SIG field,a VHT format preamble including a VHT-signal-A (VHT-SIG-A) field, a VHTshort training field (VHT-STF), a VHT long training field (VHT-LTF), anda VHT-signal-B (VHT-SIG-B) field, and a data field.

The L-STF, the L-LTF, and the L-SIG field mean legacy fields forbackward compatibility and have the same formats as those of the non-HTformat. In this case, the L-LTF may further include information forchannel estimation which will be performed in order to demodulate theL-SIG field and the VHT-SIG-A field.

The L-STF, the L-LTF, the L-SIG field, and the VHT-SIG-A field may berepeated in a 20 MHz channel unit and transmitted. For example, when aPPDU is transmitted through four 20 MHz channels (i.e., an 80 MHzbandwidth), the L-STF, the L-LTF, the L-SIG field, and the VHT-SIG-Afield may be repeated every 20 MHz channel and transmitted.

A VHT-STA may be aware of the VHT format PPDU using the VHT-SIG-A fieldsubsequent to the legacy fields, and may decode the data field based onthe VHT-SIG-A field.

In the VHT format PPDU, the L-STF, the L-LTF, and the L-SIG field arefirst transmitted so that even an L-STA can receive the VHT format PPDUand obtain data. Thereafter, the VHT-SIG-A field is transmitted for thedemodulation and decoding of data transmitted for a VHT-STA.

The VHT-SIG-A field is a field for the transmission of controlinformation that is common to a VHT STAs that are MIMO-paired with anAP, and includes control information for interpreting the received VHTformat PPDU.

The VHT-SIG-A field may include a VHT-SIG-A1 field and a VHT-SIG-A2field.

The VHT-SIG-A1 field may include information about a channel bandwidth(BW) used, information about whether space time block coding (STBC) isapplied or not, a group identifier (ID) for indicating a group ofgrouped STAs in MU-MIMO, information about the number of streams used(the number of space-time streams (NSTS)/part association identifier(AID), and transmit power save forbidden information. In this case, thegroup ID means an identifier assigned to a target transmission STA groupin order to support MU-MIMO transmission, and may indicate whether thepresent MIMO transmission method is MU-MIMO or SU-MIMO.

Table 2 illustrates the VHT-SIG-A1 field.

TABLE 2 field bit description BW 2 Set to “0” if a BW is 20 MHz Set to“1” if a BW is 40 MHz Set to “2” if a BW is 80 MHz Set to “3” if a BW is160 MHz or 80 + 80 MHz Reserved 1 STBC 1 In the case of a VHT SU PPDU:Set to “1” if STBC is used Set to “0” if not In the case of a VHT MUPPDU: Set to “0” group ID 6 Indicate a group ID “0” or “63” indicates aVHT SU PPDU, but indicates a VHT MU PPDU if not NSTS/Partial 12 In thecase of a VHT MU PPDU, divide into 4 user positions AID “p” each havingthree bits “0” if a space-time stream is 0 “1” if a space-time stream is1 “2” if a space-time stream is 2 “3” if a space-time stream is 3 “4” ifa space-time stream is 4 In the case of a VHT SU PPDU, Upper 3 bits areset as follows: “0” if a space-time stream is 1 “1” if a space-timestream is 2 “2” if a space-time stream is 3 “3” if a space-time streamis 4 “4” if a space-time stream is 5 “5” if a space-time stream is 6 “6”if a space-time stream is 7 “7” if a space-time stream is 8 Lower 9 bitsindicate a partial AID. TXOP_PS_NOT_ALLOWED 1 Set to “0” if a VHT APpermits a non-AP VHT STA to switch to power save mode duringtransmission opportunity (TXOP) Set to “1” if not In the case of a VHTPPDU transmitted by a non-AP VHT STA Set to “1” Reserved 1

The VHT-SIG-A2 field may include information about whether a short guardinterval (GI) is used or not, forward error correction (FEC)information, information about a modulation and coding scheme (MCS) fora single user, information about the type of channel coding for multipleusers, beamforming-related information, redundancy bits for cyclicredundancy checking (CRC), the tail bits of a convolutional decoder andso on.

Table 3 illustrates the VHT-SIG-A2 field.

TABLE 3 field bit description Short GI 1 Set to “0” if a short GI is notused in a data field Set to “1” if a short GI is used in a data fieldShort GI 1 Set to “1” if a short GI is used and an extra symbol isrequired disambiguation for the payload of a PPDU Set to “0” if an extrasymbol is not required SU/MU coding 1 In the case of a VHT SU PPDU: Setto “0” in the case of binary convolutional code (BCC) Set to “1” in thecase of low-density parity check (LDPC) In the case of a VHT MU PPDU:Indicate coding used if the NSTS field of a user whose user position is“0” is not “0” Set to “0” in the case of BCC Set to “1” in the case ofPDPC Set to “1” as a reserved field if the NSTS field of a user whoseuser position is “0” is “0” LDPC Extra 1 Set to “1” if an extra OFDMsymbol is required due to an PDPC OFDM symbol PPDU encoding procedure(in the case of a SU PPDU) or the PPDU encoding procedure of at leastone PDPC user (in the case of a VHT MU PPDU) Set to “0” if not SU VHT 4In the case of a VHT SU PPDU: MCS/MU coding Indicate a VHT-MCS index Inthe case of a VHT MU PPDU: Indicate coding for user positions “1” to “3”sequentially from upper bits Indicate coding used if the NSTS field ofeach user is not “1” Set to “0” in the case of BCC Set to “1” in thecase of LDPC Set to “1” as a reserved field if the NSTS field of eachuser is “0” Beamformed 1 In the case of a VHT SU PPDU: Set to “1” if abeamforming steering matrix is applied to SU transmission Set to “0” ifnot In the case of a VHT MU PPDU: Set to “1” as a reserved fieldReserved 1 CRC 8 Include CRS for detecting an error of a PPDU on thereceiver side Tail 6 Used to terminate the trellis of a convolutionaldecoder Set to “0”

The VHT-STF is used to improve AGC estimation performance in MIMOtransmission.

The VHT-LTF is used for a VHT-STA to estimate an MIMO channel. Since aVHT WLAN system supports MU-MIMO, the VHT-LTF may be configured by thenumber of spatial streams through which a PPDU is transmitted.Additionally, if full channel sounding is supported, the number ofVHT-LTFs may be increased.

The VHT-SIG-B field includes dedicated control information which isnecessary for a plurality of MU-MIMO-paired VHT-STAs to receive a PPDUand to obtain data. Accordingly, only when common control informationincluded in the VHT-SIG-A field indicates that a received PPDU is forMU-MIMO transmission, a VHT-STA may be designed to decode the VHT-SIG-Bfield. In contrast, if common control information indicates that areceived PPDU is for a single VHT-STA (including SU-MIMO), an STA may bedesigned to not decode the VHT-SIG-B field.

The VHT-SIG-B field includes a VHT-SIG-B length field, a VHT-MCS field,a reserved field, and a tail field.

The VHT-SIG-B length field indicates the length of an A-MPDU (prior toend-of-frame (EOF) padding). The VHT-MCS field includes informationabout the modulation, encoding, and rate-matching of each VHT-STA.

The size of the VHT-SIG-B field may be different depending on the type(MU-MIMO or SU-MIMO) of MIMO transmission and a channel bandwidth usedfor PPDU transmission.

FIG. 4(b) illustrates a VHT-SIG-B field according to a PPDU transmissionbandwidth.

Referring to FIG. 4(b), in 40 MHz transmission, VHT-SIG-B bits arerepeated twice. In 80 MHz transmission, VHT-SIG-B bits are repeated fourtimes, and padding bits set to 0 are attached.

In 160 MHz transmission and 80+80 MHz transmission, first, VHT-SIG-Bbits are repeated four times as in the 80 MHz transmission, and paddingbits set to 0 are attached. Furthermore, a total of the 117 bits isrepeated again.

In a system supporting MU-MIMO, in order to transmit PPDUs having thesame size to STAs paired with an AP, information indicating the size ofthe bits of a data field forming the PPDU and/or information indicatingthe size of bit streams forming a specific field may be included in theVHT-SIG-A field.

In this case, an L-SIG field may be used to effectively use a PPDUformat. A length field and a rate field which are included in the L-SIGfield and transmitted so that PPDUs having the same size are transmittedto all of STAs may be used to provide required information. In thiscase, additional padding may be required in the physical layer becausean MAC protocol data unit (MPDU) and/or an aggregate MAC PDU (A-MPDU)are set based on the bytes (or octets) of the MAC layer.

In FIG. 4, the data field is a payload and may include a service field,a scrambled PSDU, tail bits, and padding bits.

An STA needs to determine the format of a received PPDU because severalformats of PPDUs are mixed and used as described above.

In this case, the meaning that a PPDU (or a PPDU format) is determinedmay be various. For example, the meaning that a PPDU is determined mayinclude determining whether a received PPDU is a PPDU capable of beingdecoded (or interpreted) by an STA. Furthermore, the meaning that a PPDUis determined may include determining whether a received PPDU is a PPDUcapable of being supported by an STA. Furthermore, the meaning that aPPDU is determined may include determining that information transmittedthrough a received PPDU is which information.

This will be described in more detail below with reference to thedrawings.

FIG. 5 illustrates constellation diagrams for classifying a PPDU formatin a wireless communication system to which the present invention may beapplied.

(a) of FIG. 5 illustrates a constellation for the L-SIG field includedin the non-HT format PPDU, (b) of FIG. 5 illustrates a phase rotationfor HT-mixed format PPDU detection, and (c) of FIG. 5 illustrates aphase rotation for VHT format PPDU detection.

In order for an STA to classify a PPDU as a non-HT format PPDU, HT-GFformat PPDU, HT-mixed format PPDU, or VHT format PPDU, the phases ofconstellations of the L-SIG field and of the OFDM symbols, which aretransmitted following the L-SIG field, are used. That is, the STA mayclassify a PDDU format based on the phases of constellations of theL-SIG field of a received PPDU and/or of the OFDM symbols, which aretransmitted following the L-SIG field.

Referring to (a) of FIG. 5, the OFDM symbols of the L-SIG field use BPSK(Binary Phase Shift Keying).

To begin with, in order to classify a PPDU as an HT-GF format PPDU, theSTA, upon detecting a first SIG field from a received PPDU, determineswhether this first SIG field is an L-SIG field or not. That is, the STAattempts to perform decoding based on the constellation illustrated in(a) of FIG. 5. If the STA fails in decoding, the corresponding PPDU maybe classified as the HT-GF format PPDU.

Next, in order to distinguish the non-HT format PPDU, HT-mixed formatPPDU, and VHT format PPDU, the phases of constellations of the OFDMsymbols transmitted following the L-SIG field may be used. That is, themethod of modulation of the OFDM symbols transmitted following the L-SIGfield may vary, and the STA may classify a PPDU format based on themethod of modulation of fields coming after the L-SIG field of thereceived PPDU.

Referring to (b) of FIG. 5, in order to classify a PPDU as an HT-mixedformat PPDU, the phases of two OFDM symbols transmitted following theL-SIG field in the HT-mixed format PPDU may be used.

More specifically, both the phases of OFDM symbols #1 and #2corresponding to the HT-SIG field, which is transmitted following theL-SIG field, in the HT-mixed format PPDU are rotated counterclockwise by90 degrees. That is, the OFDM symbols #1 and #2 are modulated by QBPSK(Quadrature Binary Phase Shift Keying). The QBPSK constellation may be aconstellation which is rotated counterclockwise by 90 degrees based onthe BPSK constellation.

An STA attempts to decode the first and second OFDM symbolscorresponding to the HT-SIG field transmitted after the L-SIG field ofthe received PDU, based on the constellations illustrated in (b) of FIG.5. If the STA succeeds in decoding, the corresponding PPDU may beclassified as an HT format PPDU.

Next, in order to distinguish the non-HT format PPDU and the VHT formatPPDU, the phases of constellations of the OFDM symbols transmittedfollowing the L-SIG field may be used.

Referring to (c) of FIG. 5, in order to classify a PPDU as a VHT formatPPDU, the phases of two OFDM symbols transmitted after the L-SIG fieldmay be used in the VHT format PPDU.

More specifically, the phase of the OFDM symbol #1 corresponding to theVHT-SIG-A coming after the L-SIG field in the HT format PPDU is notrotated, but the phase of the OFDM symbol #2 is rotated counterclockwiseby 90 degrees. That is, the OFDM symbol #1 is modulated by BPSK, and theOFDM symbol #2 is modulated by QBPSK.

The STA attempts to decode the first and second OFDM symbolscorresponding to the VHT-SIG field transmitted following the L-SIG fieldof the received PDU, based on the constellations illustrated in (c) ofFIG. 5. If the STA succeeds in decoding, the corresponding PPDU may beclassified as a VHT format PPDU.

On the contrary, If the STA fails in decoding, the corresponding PPDUmay be classified as a non-HT format PPDU.

MAC Frame Format

FIG. 6 illustrates a MAC frame format in an IEEE 802.11 system to whichthe present invention may be applied.

Referring to FIG. 6, the MAC frame (i.e., an MPDU) includes an MACheader, a frame body, and a frame check sequence (FCS).

The MAC Header is defined as an area, including a frame control field, aduration/ID field, an address 1 field, an address 2 field, an address 3field, a sequence control field, an address 4 field, a QoS controlfield, and an HT control field.

The frame control field contains information on the characteristics ofthe MAC frame. A more detailed description of the frame control fieldwill be given later.

The duration/ID field may be implemented to have a different valuedepending on the type and subtype of a corresponding MAC frame.

If the type and subtype of a corresponding MAC frame is a PS-poll framefor a power save (PS) operation, the duration/ID field may be configuredto include the association identifier (AID) of an STA that hastransmitted the frame. In the remaining cases, the duration/ID field maybe configured to have a specific duration value depending on the typeand subtype of a corresponding MAC frame. Furthermore, if a frame is anMPDU included in an aggregate-MPDU (A-MPDU) format, the duration/IDfield included in an MAC header may be configured to have the samevalue.

The address 1 field to the address 4 field are used to indicate a BSSID,a source address (SA), a destination address (DA), a transmittingaddress (TA) indicating the address of a transmitting STA, and areceiving address (RA) indicating the address of a receiving STA.

An address field implemented as a TA field may be set as a bandwidthsignaling TA value. In this case, the TA field may indicate that acorresponding MAC frame includes additional information in a scramblingsequence. The bandwidth signaling TA may be represented as the MACaddress of an STA that sends a corresponding MAC frame, butindividual/group bits included in the MAC address may be set as aspecific value (e.g., “1”).

The sequence control field is configured to include a sequence numberand a fragment number. The sequence number may indicate a sequencenumber assigned to a corresponding MAC frame. The fragment number mayindicate the number of each fragment of a corresponding MAC frame.

The QoS control field includes information related to QoS. The QoScontrol field may be included if it indicates a QoS data frame in asubtype subfield.

The HT control field includes control information related to an HTand/or VHT transmission/reception scheme. The HT control field isincluded in a control wrapper frame. Furthermore, the HT control fieldis present in a QoS data frame having an order subfield value of 1 and amanagement frame.

The frame body is defined as an MAC payload. Data to be transmitted in ahigher layer is placed in the frame body. The frame body has a varyingsize. For example, a maximum size of an MPDU may be 11454 octets, and amaximum size of a PPDU may be 5.484 ms.

The FCS is defined as an MAC footer and used for the error search of anMAC frame.

The first three fields (i.e., the frame control field, the duration/IDfield, and Address 1 field) and the last field (i.e., the FCS field)form a minimum frame format and are present in all of frames. Theremaining fields may be present only in a specific frame type.

FIG. 7 is a diagram illustrating the frame control field in the MACframe in a wireless communication system to which the present inventionmay be applied.

Referring to FIG. 7, the frame control field includes a Protocol Versionsubfield, a Type subfield, a Subtype subfield, a to DS subfield, a FromDS subfield, a More Fragments subfield, a Retry subfield, a PowerManagement subfield, a More Data subfield, a Protected Frame subfield,and an Order subfield.

The protocol version subfield may indicate the version of a WLANprotocol applied to the MAC frame.

The type subfield and the subtype subfield may be configured to indicateinformation for identifying the function of the MAC frame.

The MAC frame may include three frame types: Management frames, Controlframes, and Data frames.

Each frame type may be subdivided into subtypes.

For example, the Control frames may include an RTS (request-to-send)frame, a CTS (clear-to-send) frame, an ACK (Acknowledgement) frame, aPS-Poll frame, a CF (contention free)-End frame, a CF-End+CF-ACK frame,a BAR (Block Acknowledgement request) frame, a BA (BlockAcknowledgement) frame, a Control Wrapper (Control+HTcontrol) frame, aVHT NDPA (Null Data Packet Announcement) frame, and a Beamforming ReportPoll frame.

The Management frames may include a Beacon frame, an ATIM (AnnouncementTraffic Indication Message) frame, a Disassociation frame, anAssociation Request/Response frame, a Reassociation Request/Responseframe, a Probe Request/Response frame, an Authentication frame, aDeauthentication frame, an Action frame, an Action No ACK frame, and aTiming Advertisement frame.

The To Ds subfield and the From DS subfield may contain informationrequired to interpret the Address 1 field through Address 4 fieldincluded in the MAC frame header. For a Control frame, the To DSsubfield and the From DS subfield may all set to ‘0’. For a Managementframe, the To DS subfield and the From DS subfield may be set to ‘1’ and‘0’, respectively, if the corresponding frame is a QoS Management frame(QMF); otherwise, the To DS subfield and the From DS subfield all may beset to ‘0’.

The More Fragments subfield may indicate whether there is a fragment tobe sent subsequent to the MAC frame. If there is another fragment of thecurrent MSDU or MMPDU, the More Fragments subfield may be set to ‘1’;otherwise, it may be set to ‘0’.

The Retry subfield may indicate whether the MAC frame is the previousMAC frame that is re-transmitted. If the MAC frame is the previous MACframe that is re-transmitted, the Retry subfield may be set to ‘1’;otherwise, it may be set to ‘0’.

The Power Management subfield may indicate the power management mode ofthe STA. If the Power Management subfield has a value of ‘1’, this mayindicate that the STA switches to power save mode.

The More Data subfield may indicate whether there is a MAC frame to beadditionally sent. If there is a MAC frame to be additionally sent, theMore Data subfield may be set to ‘1’; otherwise, it may be set to ‘0’.

The Protected Frame subfield may indicate whether a Frame Body field isencrypted or not. If the Frame Body field contains information that isprocessed by a cryptographic encapsulation algorithm, it may be set to‘1’; otherwise ‘0’.

Information contained in the above-described fields may be as defined inthe IEEE 802.11 system. Also, the above-described fields are examples ofthe fields that may be included in the MAC frame but not limited tothem. That is, the above-described fields may be substituted with otherfields or further include additional fields, and not all of the fieldsmay be necessarily included.

FIG. 8 illustrates the VHT format of an HT control field in a wirelesscommunication system to which an embodiment of the present invention maybe applied.

Referring to FIG. 8, the HT control field may include a VHT subfield, anHT control middle subfield, an AC constraint subfield, and a reversedirection grant (RDG)/more PPDU subfield.

The VHT subfield indicates whether the HT control field has the formatof an HT control field for VHT (VHT=1) or has the format of an HTcontrol field for HT (VHT=0). In FIG. 8, it is assumed that the HTcontrol field is an HT control field for VHT (i.e., VHT=1).

The HT control field for VHT may be called a VHT control field.

The HT control middle subfield may be implemented to a different formatdepending on the indication of a VHT subfield. The HT control middlesubfield is described in detail later.

The AC constraint subfield indicates whether the mapped access category(AC) of a reverse direction (RD) data frame is constrained to a singleAC.

The RDG/more PPDU subfield may be differently interpreted depending onwhether a corresponding field is transmitted by an RD initiator or an RDresponder.

Assuming that a corresponding field is transmitted by an RD initiator,the RDG/more PPDU subfield is set as “1” if an RDG is present, and theRDG/more PPDU subfield is set as “0” if an RDG is not present. Assumingthat a corresponding field is transmitted by an RD responder, theRDG/more PPDU subfield is set as “1” if a PPDU including thecorresponding subfield is the last frame transmitted by the RDresponder, and the RDG/more PPDU subfield is set as “0” if another PPDUis transmitted.

As described above, the HT control middle subfield may be implemented toa different format depending on the indication of a VHT subfield.

The HT control middle subfield of an HT control field for VHT mayinclude a reserved bit subfield, a modulation and coding scheme (MCS)feedback request (MRQ) subfield, an MRQ sequence identifier(MSI)/space-time block coding (STBC) subfield, an MCS feedback sequenceidentifier (MFSI)/least significant bit (LSB) of group ID (GID-L)subfield, an MCS feedback (MFB) subfield, a most significant Bit (MSB)of group ID (GID-H) subfield, a coding type subfield, a feedbacktransmission type (FB Tx type) subfield, and an unsolicited MFBsubfield.

Table 4 illustrates a description of each subfield included in the HTcontrol middle subfield of the VHT format.

TABLE 4 subfield meaning definition MRQ MCS request Set to “1” if MCSfeedback (solicited MFB) is not requested Set to “0” if not MSI MRQsequence An MSI subfield includes a sequence number within a identifierrange of 0 to 6 to identify a specific request if an unsolicited MFBsubfield is set to “0” and an MRQ subfield is set to “1.” Include acompressed MSI subfield (2 bits) and an STBC indication subfield (1 bit)if an unsolicited MFB subfield is “1.” MFSI/GID-L MFB sequence AnMFSI/GID-L subfield includes the received value of identifier/LSB an MSIincluded within a frame related to MFB of group ID information if anunsolicited MFB subfield is set to “0.” An MFSI/GID-L subfield includesthe lowest three bits of a group ID of a PPDU estimated by an MFB if anMFB is estimated from an MU PPDU. MFB VHT N_STS, An MFB subfieldincludes recommended MFB. MCS, BW, SNR VHT-MCS = 15, NUM_STS = 7indicates that feedback is feedback not present. GID-H MSB of group AGID-H subfield includes the most significant bit 3 bits ID of a group IDof a PPDU whose solicited MFB has been estimated if an unsolicited MFBfield is set to “1” and MFB has been estimated from a VHT MU PPDU. Allof GID-H subfields are set to “1” if MFB is estimated from an SU PPDU.Coding Type Coding type or If an unsolicited MFB subfield is set to “1”,a coding type MFB response subfield includes the coding type (binaryconvolutional code (BCC) includes 0 and low-density parity check (LDPC)includes 1) of a frame whose solicited MFB has been estimated FB Tx TypeTransmission An FB Tx Type subfield is set to “0” if an unsolicited typeof MFB MFB subfield is set to “1” and MFB has been estimated responsefrom an unbeamformed VHT PPDU. An FB Tx Type subfield is set to “1” ifan unsolicited MFB subfield is set to “1” and MFB has been estimatedfrom a beamformed VHT PPDU. Unsolicited Unsolicited Set to “1” if MFB isa response to MRQ MFB MCS feedback Set to “0” if MFB is not a responseto MRQ indicator

Furthermore, the MFB subfield may include the number of VHT space timestreams (NUM_STS) subfield, a VHT-MCS subfield, a bandwidth (BW)subfield, and a signal to noise ratio (SNR) subfield.

The NUM_STS subfield indicates the number of recommended spatialstreams. The VHT-MCS subfield indicates a recommended MCS. The BWsubfield indicates bandwidth information related to a recommended MCS.The SNR subfield indicates an average SNR value of data subcarriers andspatial streams.

The information included in each of the aforementioned fields may complywith the definition of an IEEE 802.11 system. Furthermore, each of theaforementioned fields corresponds to an example of fields which may beincluded in an MAC frame and is not limited thereto. That is, each ofthe aforementioned fields may be substituted with another field,additional fields may be further included, and all of the fields may notbe essentially included.

Medium Access Mechanism

In IEEE 802.11, communication is basically different from that of awired channel environment because it is performed in a shared wirelessmedium.

In a wired channel environment, communication is possible based oncarrier sense multiple access/collision detection (CSMA/CD). Forexample, when a signal is once transmitted by a transmission stage, itis transmitted up to a reception stage without experiencing great signalattenuation because there is no great change in a channel environment.In this case, when a collision between two or more signals is detected,detection is possible. The reason for this is that power detected by thereception stage becomes instantly higher than power transmitted by thetransmission stage. In a radio channel environment, however, sincevarious factors (e.g., signal attenuation is great depending on thedistance or instant deep fading may be generated) affect a channel, atransmission stage is unable to accurately perform carrier sensingregarding whether a signal has been correctly transmitted by a receptionstage or a collision has been generated.

Accordingly, in a WLAN system according to IEEE 802.11, a carrier sensemultiple access with collision avoidance (CSMA/CA) mechanism has beenintroduced as the basic access mechanism of MAC. The CAMA/CA mechanismis also called a distributed coordination function (DCF) of IEEE 802.11MAC, and basically adopts a “listen before talk” access mechanism. Inaccordance with such a type of access mechanism, an AP and/or an STAperform clear channel assessment (CCA) for sensing a radio channel or amedium for a specific time interval (e.g., a DCF inter-frame space(DIFS)) prior to transmission. If, as a result of the sensing, themedium is determined to be an idle state, the AP and/or the STA startsto transmit a frame through the corresponding medium. In contrast, if,as a result of the sensing, the medium is determined to be a busy state(or an occupied status), the AP and/or the STA do not start theirtransmission, may wait for a delay time (e.g., a random backoff period)for medium access in addition to the DIFS assuming that several STAsalready wait for in order to use the corresponding medium, and may thenattempt frame transmission.

Assuming that several STAs trying to transmit frames are present byapplying the random backoff period, they will wait for different timesbecause the STAs stochastically have different backoff period values andwill attempt frame transmission. In this case, a collision can beminimized by applying the random backoff period.

Furthermore, the IEEE 802.11 MAC protocol provides a hybrid coordinationfunction (HCF). The HCF is based on a DCF and a point coordinationfunction (PCF). The PCF is a polling-based synchronous access method,and refers to a method for periodically performing polling so that allof receiving APs and/or STAs can receive a data frame. Furthermore, theHCF has enhanced distributed channel access (EDCA) and HCF controlledchannel access (HCCA). In EDCA, a provider performs an access method forproviding a data frame to multiple users on a contention basis. In HCCA,a non-contention-based channel access method using a polling mechanismis used. Furthermore, the HCF includes a medium access mechanism forimproving the quality of service (QoS) of a WLAN, and may transmit QoSdata in both a contention period (CP) and a contention-free period(CFP).

FIG. 9 is a diagram illustrating a random backoff period and a frametransmission procedure in a wireless communication system to which anembodiment of the present invention may be applied.

When a specific medium switches from an occupied (or busy) state to anidle state, several STAs may attempt to transmit data (or frames). Inthis case, as a scheme for minimizing a collision, each of the STAs mayselect a random backoff count, may wait for a slot time corresponding tothe selected random backoff count, and may attempt transmission. Therandom backoff count has a pseudo-random integer value and may bedetermined as one of uniformly distributed values in 0 to a contentionwindow (CW) range. In this case, the CW is a CW parameter value. In theCW parameter, CW_min is given as an initial value. If transmission fails(e.g., if ACK for a transmitted frame is not received), the CW_min mayhave a twice value. If the CW parameter becomes CW_max, it may maintainthe CW_max value until data transmission is successful, and the datatransmission may be attempted. If the data transmission is successful,the CW parameter is reset to a CW_min value. The CW, CW_min, and CW_maxvalues may be set to 2{circumflex over ( )}n−1 (n=0, 1, 2, . . . ,).

When a random backoff process starts, an STA counts down a backoff slotbased on a determined backoff count value and continues to monitor amedium during the countdown. When the medium is monitored as a busystate, the STA stops the countdown and waits. When the medium becomes anidle state, the STA resumes the countdown.

In the example of FIG. 9, when a packet to be transmitted in the MAC ofan STA 3 is reached, the STA 3 may check that a medium is an idle stateby a DIFS and may immediately transmit a frame.

The remaining STAs monitor that the medium is the busy state and wait.In the meantime, data to be transmitted by each of an STA 1, an STA 2,and an STA 5 may be generated. When the medium is monitored as an idlestate, each of the STAs waits for a DIFS and counts down a backoff slotbased on each selected random backoff count value.

The example of FIG. 9 shows that the STA 2 has selected the smallestbackoff count value and the STA 1 has selected the greatest backoffcount value. That is, FIG. 7 illustrates that the remaining backoff timeof the STA 5 is shorter than the remaining backoff time of the STA 1 ata point of time at which the STA 2 finishes a backoff count and startsframe transmission.

The STA 1 and the STA 5 stop countdown and wait while the STA 2 occupiesthe medium. When the occupation of the medium by the STA 2 is finishedand the medium becomes an idle state again, each of the STA 1 and theSTA 5 waits for a DIFS and resumes the stopped backoff count. That is,each of the STA 1 and the STA 5 may start frame transmission aftercounting down the remaining backoff slot corresponding to the remainingbackoff time. The STA 5 starts frame transmission because the STA 5 hasa shorter remaining backoff time than the STA 1.

While the STA 2 occupies the medium, data to be transmitted by an STA 4may be generated. In this case, from a standpoint of the STA 4, when themedium becomes an idle state, the STA 4 waits for a DIFS and counts downa backoff slot corresponding to its selected random backoff count value.

FIG. 9 shows an example in which the remaining backoff time of the STA 5coincides with the random backoff count value of the STA 4. In thiscase, a collision may be generated between the STA 4 and the STA 5. Whena collision is generated, both the STA 4 and the STA 5 do not receiveACK, so data transmission fails. In this case, each of the STA 4 and theSTA 5 doubles its CW value, select a random backoff count value, andcounts down a backoff slot.

The STA 1 waits while the medium is the busy state due to thetransmission of the STA 4 and the STA 5. When the medium becomes an idlestate, the STA 1 may wait for a DIFS and start frame transmission afterthe remaining backoff time elapses.

The CSMA/CA mechanism includes virtual carrier sensing in addition tophysical carrier sensing in which an AP and/or an STA directly sense amedium.

Virtual carrier sensing is for supplementing a problem which may begenerated in terms of medium access, such as a hidden node problem. Forthe virtual carrier sensing, the MAC of a WLAN system uses a networkallocation vector (NAV). The NAV is a value indicated by an AP and/or anSTA which now uses a medium or has the right to use the medium in orderto notify another AP and/or STA of the remaining time until the mediumbecomes an available state. Accordingly, a value set as the NAVcorresponds to the period in which a medium is reserved to be used by anAP and/or an STA that transmit corresponding frames. An STA thatreceives an NAV value is prohibited from accessing the medium during thecorresponding period. The NAV may be set based on the value of theduration field of the MAC header of a frame, for example.

An AP and/or an STA may perform a procedure for exchanging a request tosend (RTS) frame and a clear to send (CTS) frame in order to providenotification that they will access a medium. The RTS frame and the CTSframe include information indicating a temporal section in which awireless medium required to transmit/receive an ACK frame has beenreserved to be accessed if substantial data frame transmission and anacknowledgement response (ACK) are supported. Another STA which hasreceived an RTS frame from an AP and/or an STA attempting to send aframe or which has received a CTS frame transmitted by an STA to which aframe will be transmitted may be configured to not access a mediumduring a temporal section indicated by information included in theRTS/CTS frame. This may be implemented by setting the NAV during a timeinterval.

Interframe Space (IFS)

A time interval between frames is defined as an interframe space (IFS).An STA may determine whether a channel is used during an IFS timeinterval through carrier sensing. In an 802.11 WLAN system, a pluralityof IFSs is defined in order to provide a priority level by which awireless medium is occupied.

FIG. 10 is a diagram illustrating an IFS relation in a wirelesscommunication system to which an embodiment of the present invention maybe applied.

All of pieces of timing may be determined with reference to physicallayer interface primitives, that is, a PHY-TXEND.confirm primitive, aPHYTXSTART.confirm primitive, a PHY-RXSTART.indication primitive, and aPHY-RXEND.indication primitive.

An interframe space (IFS) depending on an IFS type is as follows.

a) A reduced interframe space (IFS) (RIFS)

b) A short interframe space (IFS) (SIFS)

c) A PCF interframe space (IFS) (PIFS)

d) A DCF interframe space (IFS) (DIFS)

e) An arbitration interframe space (IFS) (AIFS)

f) An extended interframe space (IFS) (EIFS)

Different IFSs are determined based on attributes specified by aphysical layer regardless of the bit rate of an STA. IFS timing isdefined as a time gap on a medium. IFS timing other than an AIFS isfixed for each physical layer.

The SIFS is used to transmits a PPDU including an ACK frame, a CTSframe, a block ACK request (BlockAckReq) frame, or a block ACK(BlockAck) frame, that is, an instant response to an A-MPDU, the secondor consecutive MPDU of a fragment burst, and a response from an STA withrespect to polling according to a PCF. The SIFS has the highestpriority. Furthermore, the SIFS may be used for the point coordinator offrames regardless of the type of frame during a non-contention period(CFP) time. The SIFS indicates the time prior to the start of the firstsymbol of the preamble of a next frame which is subsequent to the end ofthe last symbol of a previous frame or from signal extension (ifpresent).

SIFS timing is achieved when the transmission of consecutive frames isstarted in a Tx SIFS slot boundary.

The SIFS is the shortest in IFS between transmissions from differentSTAs. The SIFS may be used if an STA occupying a medium needs tomaintain the occupation of the medium during the period in which theframe exchange sequence is performed.

Other STAs required to wait so that a medium becomes an idle state for alonger gap can be prevented from attempting to use the medium becausethe smallest gap between transmissions within a frame exchange sequenceis used. Accordingly, priority may be assigned in completing a frameexchange sequence that is in progress.

The PIFS is used to obtain priority in accessing a medium.

The PIFS may be used in the following cases.

-   -   An STA operating under a PCF    -   An STA sending a channel switch announcement frame    -   An STA sending a traffic indication map (TIM) frame    -   A hybrid coordinator (HC) starting a CFP or transmission        opportunity (TXOP)    -   An HC or non-AP QoS STA, that is, a TXOP holder polled for        recovering from the absence of expected reception within a        controlled access phase (CAP)    -   An HT STA using dual CTS protection before sending CTS2    -   A TXOP holder for continuous transmission after a transmission        failure    -   A reverse direction (RD) initiator for continuous transmission        using error recovery    -   An HT AP during a PSMP sequence in which a power save multi-poll        (PSMP) recovery frame is transmitted    -   An HT AT performing CCA within a secondary channel before        sending a 40 MHz mask PPDU using EDCA channel access

In the illustrated examples, an STA using the PIFS starts transmissionafter a carrier sense (CS) mechanism for determining that a medium is anidle state in a Tx PIFS slot boundary other than the case where CCA isperformed in a secondary channel.

The DIFS may be used by an STA which operates to send a data frame(MPDU) and a MAC management protocol data unit management (MMPDU) frameunder the DCF. An STA using the DCF may transmit data in a TxDIFS slotboundary if a medium is determined to be an idle state through a carriersense (CS) mechanism after an accurately received frame and a backofftime expire. In this case, the accurately received frame means a frameindicating that the PHY-RXEND.indication primitive does not indicate anerror and an FCS indicates that the frame is not an error (i.e., errorfree).

An SIFS time (“aSIFSTime”) and a slot time (“aSlotTime”) may bedetermined for each physical layer. The SIFS time has a fixed value, butthe slot time may be dynamically changed depending on a change in thewireless delay time “aAirPropagationTime.”

The “aSIFSTime” is defined as in Equations 1 and 2 below.

aSIFSTime(16μs)=aRxRFDelay(0.5)+aRxPLCPDelay(12.5)+aMACProcessingDelay(1 or<2)+aRxTxTurnaroundTime(<2)  [Equation 1]

aRxTxTurnaroundTime=aTxPLCPDelay(l)+aRxTxSwitchTime(0.25)+aTxRamoOnTime(0.25)+aTxRFDelay(0.5)  [Equation2]

The “aSlotTime” is defined as in Equation 3 below.

aSlotTime=aCCATime(<4)+aRxTxTurnaroundTime(<2)+eAirProPagationTime(<1)+aMACProoessingDelay(<2)  [Equation3]

In Equation 3, a default physical layer parameter is based on“aMACProcessingDelay” having a value which is equal to or smaller than 1μs. A radio wave is spread 300 m/μs in the free space. For example, 3 μsmay be the upper limit of a BSS maximum one-way distance ˜450 m (a roundtrip is −900 m).

The PIFS and the SIFS are defined as in Equations 4 and 5, respectively.

PIFS(16 μs)=aSIFSTime+aSlotTime  [Equation 4]

DIFS(34 μs)=aSIFSTime+2 aSlotTime  [Equation 5]

In Equations 1 to 5, the numerical value within the parenthesisillustrates a common value, but the value may be different for each STAor for the position of each STA.

The aforementioned SIFS, PIFS, and DIFS are measured based on an MACslot boundary (e.g., a Tx SIFS, a Tx PIFS, and a TxDIFS) different froma medium.

The MAC slot boundaries of the SIFS, the PIFS, and the DIFS are definedas in Equations 6 to 8, respectively.

TxSIFS=SIFS−aRxTxTurnaroundTime  [Equation 6]

TxPIFS=TxSIFS+aSlotTime  [Equation 7]

TxDIFS=TxSIFS+2*aSlotTime  [Equation 8]

Channel State Information Feedback Method

SU-MIMO technology, in which a beamformer assigns all antennas to onebeamformee for communication, enhances channel capacity throughspatial-temporal diversity gain and multi-stream transmission. SU-MIMOtechnology uses more antennas than when MIMO technology is not used,thereby leveraging spatial degrees of freedom and contributing to theimprovement of a physical layer.

MU-MIMO technology, in which a beamformer assigns antennas to multiplebeamforms, can improve the performance of MIMO antennas by increasingthe per-beamformee transfer rate or channel reliability through a linklayer protocol for multiple access of multiple beamformees connected tothe beamformer.

In MIMO environments, performance depends largely on how accuratechannel information the beamformer acquires. Thus, a feedback procedureis required to acquire channel information.

There are largely two types of feedback supported to acquire channelinformation: one is to use a control frame and the other is to use achannel sounding procedure which does not include a data field. Soundingrefers to using a preamble training field to measure channel for otherpurposes than data demodulation of a PPDU including the correspondingtraining field.

Hereinafter, a channel information feedback method using a control frameand a channel information feedback method using an NDP (null datapacket) will be described in more detail.

1) Feedback Using Control Frame

In MIMO environments, a beamformer may instruct a beamformee to sendchannel state information feedback through the HT control field includedin the MAC header, or the beamformee may report channel stateinformation through the HT control field included in the MAC header (seeFIG. 8). The HT control field may be included in a Control Wrapperframe, a QoS Data frame in which the Order subfield of the MAC header isset to 1, and a Management frame.

2) Feedback Using Channel Sounding

FIG. 11 is a diagram conceptually showing a method of channel soundingin a wireless communication system to which the present invention may beapplied.

FIG. 11 illustrates a method of feedback of channel state informationbetween a beamformer (e.g., AP) and a beamformee (e.g., non-AP STA)based on a sounding protocol. The sounding protocol may refer to aprocedure of receiving feedback about information on channel stateinformation.

A method of sounding channel state information between a beamformer anda beamformee based on a sounding protocol may be performed in thefollowing steps:

(1) A beamformer transmits a VHT NDPA (VHT Null Data PacketAnnouncement) frame indicating sounding and transmission for feedbackfrom a beamformee.

The VHT NDPA frame refers to a control frame that is used to indicatethat channel sounding is initiated and an NDP (Null Data Packet) istransmitted. In other words, a VHT NDPA frame may be transmitted beforeNDP transmission to allow a beamformee to ready to feed back channelstate information before receiving the NDP frame.

The VHT NDPA frame may contain AID (association identifier) information,feedback type information, etc. of a beamformee that will transmit anNDP. A more detailed description of the VHT NDPA frame will be givenlater.

The VHT NDPA frame may be transmitted in different ways forMU-MIMO-based data transmission and SU-MIMO-based data transmission. Forexample, in the case of channel sounding for MU-MIMO, the VHT NDPA framemay be transmitted in a broadcast manner, whereas, in the case ofchannel sounding for SU-MIMO, the VHT NDPA frame may be transmitted in aunicast manner.

(2) After transmitting the VHT NDPA frame, the beamformer transmits anNDP after an SIFS. The NDP has a VHT PPDU structure but without a datafield.

Beamformees that have received the VHT NDPA frame may check the value ofthe AID12 subfield included in the STA information field and determinewhether they are a target STA for sounding.

Moreover, the beamformees may know their order of feedback through theSTA Info field included in the NDPA. FIG. 11 illustrates that feedbackoccurs in the order of Beamformee 1, Beamformee 2, and Beamformee 3.

(3) Beamformee 1 acquires downlink channel state information based onthe training field included in the NDP and generates feedbackinformation to send to the beamformer.

Beamformee 1 transmits a VHT compressed beamforming frame containingfeedback information to the beamformer after an SIFS after receiving theNDP frame.

The VHT compressed beamforming frame may include an SNR value for aspace-time stream, information on a compressed beamforming feedbackmatrix for a subcarrier, and so on. A more detailed description of theVHT compressed beamforming frame will be provided later.

(4) The beamformer receives the VHT compressed beamforming frame fromBeamformee 1, and then, after an SIFS, transmits a Beamforming ReportPoll frame to Beamformee 2 in order to acquire channel information fromBeamformee 2.

The Beamforming Report Poll frame is a frame that performs the same roleas the NDP frame. Beamformee 2 may measure channel state based on thetransmitted Beamforming Report Poll frame.

A more detailed description of the Beamforming Report Poll frame will begiven later.

(5) After receiving the Beamforming Report Poll frame, Beamformee 2transmits a VHT Compressed Beamforming frame containing feedbackinformation to the beamformer after an SIFS.

(6) The beamformer receives the VHT Compressed Beamforming frame fromBeamformee 2 and then, after an SIFS, transmits a Beamforming ReportPoll frame to Beamformee 3 in order to acquire channel information fromBeamformee 3.

(7) After receiving the Beamforming Report Poll frame, Beamformee 3transmits a VHT Compressed Beamforming frame containing feedbackinformation to the beamformer after an SIFS.

Hereinafter, a frame used for the above-described channel soundingprocedure will be discussed.

FIG. 12 is a diagram illustrating a VHT NDPA frame in a wirelesscommunication system to which the present invention may be applied.

Referring to FIG. 12, a VHT NDPA frame may consist of a Frame Controlfield, a Duration field, an RA (Receiving Address) field, a TA(Transmitting Address) field, a Sounding Dialog Token field, an STA Info1 field through STA info n field, and an FCS.

The RA field value indicates the address of a receiver or STA whichreceives the VHT NDPA frame.

If the VHT NDPA frame includes only one STA Info field, then the RAfield is set to the address of the STA identified by the AID in the STAInfo field. For example, when transmitting the VHT NDPA frame to onetarget STA for SU-MIMO channel sounding, an AP unicasts the VHT NDPAframe to the target STA.

On the other hand, if the VHT NDPA frame includes more than one STA Infofield, then the RA field is set to the broadcast address. For example,when transmitting the VHT NDPA frame to at least one target STA forMU-MIMO channel sounding, an AP broadcasts the VHT NDPA frame.

The TA field value indicates the address of a transmitter ortransmitting STA which transmits the VHT NDPA frame or a bandwidthsignaling TA.

The Sounding Dialog Token field also may be called a Sounding Sequencefield. The Sounding Dialog Token Number subfield in the Sounding DialogToken field contains a value selected by the beamformer to identify theVHT NDPA frame.

The VHT NDPA frame includes at least one STA Info field. That is, theVHT NDPA frame includes an STA Info field containing information ontarget STAs for sounding. One STA Info field may be included for eachtarget STA for sounding.

Each STA Info field may include an AID12 subfield, a Feedback Typesubfield, and an NC Index subfield.

Table 5 shows the subfields of an STA Info field included in the VHTNDPA frame.

TABLE 5 Subfield Description AID12 Contains the AID of a target STA forsounding feedback. The AID12 subfield value is set to ‘0’ if the targetSTA is an AP, mesh STA, or STA that is a member of an IBSS. FeedbackIndicates the type of feedback requested for the target Type STA forsounding. Set to 0 for SU-MIMO. Set to 1 for MU-MIMO. Nc Index If theFeedback Type subfield indicates MU-MIMO, then NcIndex indicates thenumber of columns, Nc, in the Compressed Beamforming Feedback Matrixsubfield minus 1. Set to 0 for Nc = 1, Set to 1 for Nc = 2, . . . Set to7 for Nc = 8. Reserved if the Feedback Type subfield indicates SU-MIMO.

Information contained in the above-described fields may be as defined inthe IEEE 802.11 system. Also, the above-described fields are examples ofthe fields that may be included in the MAC frame but not limited tothem. That is, the above-described fields may be substituted with otherfields or further include additional fields.

FIG. 13 is a diagram illustrating an NDP PPDU in a wirelesscommunication system to which the present invention may be applied.

Referring to FIG. 13, an NDP may have the VHT PPDU format shownpreviously in FIG. 4, but without the data field. The NDP may beprecoded based on a particular precoding matrix and transmitted to atarget STA for sounding.

In the L-SIG field of the NDP, the length field indicating the length ofa PSDU included in the data field is set to ‘0’.

In the VHT-SIG-A field of the NDP, the Group ID field indicating whethera transmission technique used for NDP transmission is MU-MIMO or SU-MIMOis set to a value indicating SU-MIMO transmission.

The data bits of the VHT-SIG-B field of the NDP are set to a fixed bitpattern for each bandwidth.

Upon receiving the NDP, the target STA for sounding performs channelestimation and acquires channel state information.

FIG. 14 is a diagram illustrating a VHT compressed beamforming frameformat in a wireless communication system to which the present inventionmay be applied.

Referring to FIG. 14, the VHT compressed beamforming frame is a VHTAction frame for supporting VHT functionality, and its frame bodyincludes an Action field. The Action field is included in the frame bodyof a MAC frame to provide a mechanism for specifying extended managementactions.

The Action field consists of a Category field, a VHT Action field, a VHTMIMO Control field, a VHT Compressed Beamforming Report field, and an MUExclusive Beamforming Report field.

The Category field is set to a value indicating the VHT category (i.e.,VHT Action frame), and the VHT Action field is set to a value indicatingthe VHT Compressed Beamforming frame.

The VHT MIMO Control field is used to feed back control informationrelated to beamforming feedback. The VHT MIMO Control field may alwaysbe present in the VHT Compressed Beamforming frame.

The VHT Compressed Beamforming Report field is used to feed backinformation on a beamforming matrix containing SNR information forspace-time streams used for transmitting data.

The MU Exclusive Beamforming Report field is used to feed back SNRinformation for spatial streams when performing a MU-MIMO transmission.

The presence and content of the VHT Compressed Beamforming Report fieldand the MU Exclusive Beamforming Report field are dependent on thevalues of the Feedback Type, Remaining Feedback Segments, and FirstFeedback Segment subfields of the VHT MIMO Control field.

Hereinafter, the VHT MIMO Control field, the VHT Compressed BeamformingReport field, and the MU Exclusive Beamforming Report field may bediscussed more concretely.

1) The VHT MIMO Control field consists of an Nc index subfield, an NrIndex subfield, a Channel Width subfield, a Grouping subfield, aCodebook Information subfield, a Feedback type subfield, a RemainingFeedback segments subfield, a First Feedback segment subfield, areserved subfield, and a Sounding Dialog Token Number field.

Table 6 shows the subfields of the VHT MIMO Control field.

TABLE 6 Subfield Bits Description Nc Index 3 Indicates the number ofcolumns, Nc, in the compressed beamforming feedback matrix minus 1: Setto 0 for Nc = 1, Set to 1 for Nc = 2, . . . Set to 7 for Nc = 8. NrIndex 3 Indicates the number of rows, Nr, in the compressed beamformingfeedback matrix minus 1: Set to 0 for Nr = 1, Set to 1 for Nr = 2, . . .Set to 7 for Nr = 8. Channel Width 2 Indicates the width of the channelmeasured to create a compressed beamforming feedback matrix: Set to 0for 20 MHz, Set to 1 for 40 MHz, Set to 2 for 80 MHz, Set to 3 for 160MHz or 80 + 80 MHz. Grouping 2 Indicates the subcarrier grouping, Ng,used for the compressed beamforming feedback matrix: Set to 0 for Ng = 1(No grouping), Set to 1 for Ng = 2, Set to 2 for Ng = 4, The value 3 isreserved. Codebook 1 Indicates the size of codebook entries: InformationIf Feedback Type is SU: Set to 0 for bΨ = 2 and bΦ = 4, Set to 1 for bΨ= 4 and bΦ = 6. If Feedback Type is MU: Set to 0 for bΨ = 5 and bΦ = 7Set to 1 for bΨ = 7 and bΦ = 9. Here, bΨ and bΦ indicate the number ofquantization bits. Feedback Type 1 Indicates the feedback type: Set to 0for SU-MIMO, Set to 1 for MU-MIMO. Remaining 3 Indicates the number ofremaining feedback segments for the Feedback associated VHT CompressedBeamforming frame: Segments Set to 0 for the last feedback segment of asegmented report or the only feedback segment of an unsegmented report.Set to a value between 1 and 6 for a feedback segment that is neitherthe first nor the last of a segmented report. Set to a value between 1and 6 for a feedback segment that is not the last feedback segment of asegmented report. In a retransmitted feedback segment, the field is setto the same value as the associated feedback segment in the originaltransmission. First Feedback 1 Set to 1 for the first feedback segmentof a segmented report Segment or the only feedback segment of anunsegmented report; Set to 0 if it is not the first feedback segment orif the VHT Compressed Beamforming Report field and MU ExclusiveBeamforming Report field are not present in the frame. In aretransmitted feedback segment, the field is set to the same value asthe associated feedback segment in the original transmission. SoundingDialog 6 Set to the value of the sounding dialog token of the NDPA TokenNumber frame.

In a VHT Compressed Beamforming frame not carrying all or part of theVHT Compressed Beamforming Report field, the Nc Index subfield, Nr Indexsubfield, Channel Width subfield, Grouping subfield, CodebookInformation subfield, Feedback Type subfield, and Sounding Dialog TokenNumber field are reserved, the First Feedback Segment field is set to 0,and the Remaining Feedback Segments field is set to 7.

The Sounding Dialog Token Number field also may be called a SoundingSequence Number subfield.

2) The VHT Compressed Beamforming Report field is used to carry explicitfeedback information in the form of angles representing compressedbeamforming feedback matrices V for use by a transmit beamformer todetermine steering matrices Q.

Table 7 shows the subfields of the VHT Compressed Beamforming Reportfield.

TABLE 7 Subfield Bits Description Average SNR of Space- 8Signal-to-noise ratio at the beamformee for Time Stream 1 space-timestream 1 averaged over all subcarriers . . . . . . . . . Average SNR ofSpace- 8 Signal-to-noise ratio at the beamformee for Time Stream Ncspace-time stream Nc averaged over all subcarriers Compressed Na × (bΨ +bΦ)/2 Order of angles in the Compressed Beamforming Feedback Beamformingfeedback matrix for the Matrix V for subcarrier corresponding subcarrierk = scidx(0) Compressed Na × (bΨ + bΦ)/2 Order of angles in theCompressed Beamforming Feedback Beamforming feedback matrix for theMatrix V for subcarrier corresponding subcarrier k = scidx(1) . . . . .. . . . Compressed Na × (bΨ + bΦ)/2 Order of angles in the CompressedBeamforming Feedback Beamforming feedback matrix for the Matrix V forsubcarrier corresponding subcarrier k = scidx(Ns − 1)

With reference to Table 7, the VHT compressed beamforming report fieldmay include the average SNR of each space-time stream and a CompressedBeamforming Feedback Matrix V for each subcarrier. The CompressedBeamforming Feedback Matrix is a matrix including information aboutchannel state and can be used to calculate a channel matrix (i.e.,steering matrix Q) for an MIMO-based transmission method.

scidx( ) refers to subcarriers which transmit the Compressed BeamformingFeedback Matrix subfield. Na is fixed by the Nr×Nc value (e.g., Φ11,Ψ21, . . . for Nr×Nc=2×1).

Ns refers to the number of subcarriers which transmit a compressedbeamforming feedback matrix to the beamformer. A beamformee, by using agrouping method, can reduce the number of subcarriers Ns which transmitthe compressed beamforming feedback matrix. For example, the number ofbeamforming feedback matrices provided as feedback information can bereduced by grouping a plurality of subcarriers into one group andtransmitting a compressed beamforming feedback matrix for thecorresponding group. Ns may be calculated from the Channel Width andGrouping subfields in the VHT MIMO Control field.

Table 8 illustrates the average SNR of Space-Time Stream subfield.

TABLE 8 Average SNR of Space-Time i subfield AvgSNR_i −128 ≤10 dB −127−9.75 dB −126 −9.5 dB . . . . . . +126 53.5 dB +127 ≥53.75 dB

With reference to Table 8, an average SNR for each stream-space streamis obtained by calculating the average SNR of all subcarriers in thecorresponding channel and mapping the calculated average SNR into therange of −128 to +128.

3) The MU Exclusive Beamforming Report field is used to carry explicitfeedback information in the form of delta ( ) SNRs. The information inthe VHT Compressed Beamforming Report field and the MU ExclusiveBeamforming Report field can be used by an MU beamformer to determinesteering matrices Q.

Table 9 shows the subfields of the MU Exclusive Beamforming Report fieldincluded in a VHT compressed beamforming frame.

TABLE 9 Subfield Bits Description Delta SNR for space-time 4 Thedeviation between the SNR of the corresponding stream 1 for subcarrier k= subcarrier and the average SNR of all subcarriers for sscidx(0) thecorresponding space-time stream. . . . . . . Delta SNR for space-time 4The deviation between the SNR of the corresponding stream Nc forsubcarrier k = subcarrier and the average SNR of all subcarriers forsscidx(0) the corresponding space-time stream. . . . . . . Delta SNR forspace-time 4 The deviation between the SNR of the corresponding stream 1for subcarrier k = subcarrier and the average SNR of all subcarriers forsscidx(1) the corresponding space-time stream. . . . . . . Delta SNR forspace-time 4 The deviation between the SNR of the corresponding streamNc for subcarrier k = subcarrier and the average SNR of all subcarriersfor sscidx(1) the corresponding space-time stream. . . . . . . Delta SNRfor space-time 4 The deviation between the SNR of the correspondingstream 1 for subcarrier k = subcarrier and the average SNR of allsubcarriers for sscidx(Ns′ − 1) the corresponding space-time stream. . .. . . . Delta SNR for space-time 4 The deviation between the SNR of thecorresponding stream Nc for subcarrier k = subcarrier and the averageSNR of all subcarriers for sscidx(Ns′ − 1) the corresponding space-timestream.

With reference to Table 9, the MU Exclusive Beamforming Report field mayinclude an SNR for each space-time stream for each subcarrier.

Each Delta SNR subfield has a value which is in the range −8 dB to 7 dBin 1 dB increments.

scidx( ) refers to subcarrier(s) which transmit the Delta SNR subfield.Ns refers to the number of subcarriers which transmit the Delta SNRsubfield to the beamformer.

FIG. 15 is a diagram illustrating a Beamforming Report Poll frame formatin a wireless communication system to which the present invention may beapplied.

Referring to FIG. 15, the Beamforming Report Poll frame consists of aFrame Control field, a Duration field, an RA (Receiving Address) field,a TA (Transmitting Address) field, a Feedback Segment RetransmissionBitmap field, and an FCS.

The RA field value is the address of the intended recipient.

The TA field value is the address of the STA transmitting theBeamforming Report Poll or a bandwidth signaling TA.

The Feedback Segment Retransmission Bitmap field indicates the requestedfeedback segments of a VHT Compressed Beamforming report.

If the bit in position n (n=0 for LSB and n=7 for MSB) is 1, then thefeedback segment with the Remaining Feedback Segments subfield in theVHT MIMO Control field equal to n is requested. If the bit in position nis 0, then the feedback segment with the Remaining Feedback Segmentssubfield in the VHT MIMO Control field equal to n is not requested.

Group ID

Since a VHT WLAN system supports MU-MIMO transmission for higherthroughput, an AP may transmit a data frame simultaneously to at leastone MIMO-paired STA. The AP may transmit data simultaneously to an STAgroup including at least one STA associated with it. For example, themaximum number of paired STAs may be 4. When the maximum number ofspatial streams is 8, up to 4 spatial streams may be allotted to eachSTA.

In a WLAN system supporting Tunneled Direct Link Setup (TDLS), DirectLink Setup (DLS), or a mesh network, an STA trying to send data may senda PPDU to a plurality of STAs by using the MU-MIMO transmission scheme.

An example in which an AP sends a PPDU to a plurality of STAs accordingto the MU-MIMO transmission scheme is described below.

An AP transmits a PPDU simultaneously to paired STAs belonging to atransmission target STA group through different spatial streams. Asdescribed above, the VHT-SIG-A field of the VHT PPDU format includesGroup ID information and space-time stream information. Thus, each STAmay determine whether a PPDU is sent to itself. No spatial streams maybe assigned to particular STAs in the transmission target STA group andtherefore no data will be transmitted.

A Group ID Management frame is used to assign or change a user positioncorresponding to one or more group IDs. That is, the AP may inform ofSTAs connected to a particular group ID through the Group ID Managementframe before performing a MU-MIMO transmission.

FIG. 16 is a diagram illustrating a Group ID Management frame in awireless communication system to which the present invention may beapplied.

Referring to FIG. 16, the Group ID Management frame is a VHT Actionframe for supporting VHT functionality, and its frame body includes anAction field. The Action field is included in the frame body of a MACframe to provide a mechanism for specifying extended management actions.

The Action field consists of a Category field, a VHT Action field, a VHTMIMO Control field, a Membership Status Array field, and a User PositionArray field.

The Category field is set to a value indicating the VHT category (i.e.,VHT Action frame), and the VHT Action field is set to a value indicatingthe Group ID Management frame.

The Membership Status Array field consists of a 1-bit Membership Statussubfield for each group. If the Membership Status subfield is set to 0,this indicates that the STA is not a member of the group, and if theMembership Status subfield is set to 1, this indicates that the STA is amember of the group. By setting one or more Membership Status subfieldsin the Membership Status Array field to 1, one or more groups may beassigned to the STA.

The STA may have a user position in each group to which it belongs.

The User Position Array field consists of a 2-bit User Position subfieldfor each group. The user position of an STA in a group to which itbelongs is indicated by the User Position subfield in the User PositionArray field. An AP may assign the same user position to different STAsin each group.

An AP may transmit a Group ID Management frame only if thedot11VHTOptionImplemented parameter is true. The Group ID Managementframe shall be sent only to VHT STAs that have the MU Beamformee Capablefield in the VHT Capabilities element field set to 1. The Group IDManagement frame shall be sent as an individually addressed frame.

An STA receives a Group ID Management frame with an RA field matchingits MAC address. The STA updates GROUP_ID_MANAGEMENT, a PHYCONFIG_VECTORparameter, based on the content of the received Group ID Managementframe.

Transmission of a Group ID Management frame to a STA and any associatedacknowledgement from the STA shall be complete before the transmissionof an MU PPDU to the STA.

An MU PPDU shall be transmitted to a STA based on the content of theGroup ID Management frame that is most recently transmitted to the STAand for which an ACK is received.

Downlink (DL) MU-MIMO Frame

FIG. 17 is a diagram illustrating a DL multi-user (MU) PPDU format in awireless communication system to which an embodiment of the presentinvention may be applied.

Referring to FIG. 17, the PPDU is configured to include a preamble and adata field. The data field may include a service field, a scrambled PSDUfield, tail bits, and padding bits.

An AP may aggregate MPDUs and transmit a data frame using an aggregatedMPDU (A-MPDU) format. In this case, a scrambled PSDU field may includethe A-MPDU.

The A-MPDU includes a sequence of one or more A-MPDU subframes.

In the case of a VHT PPDU, the length of each A-MPDU subframe is amultiple of 4 octets. Accordingly, an A-MPDU may include an end-of-frame(EOF) pad of 0 to 3 octets after the last A-MPDU subframe in order tomatch the A-MPDU up with the last octet of a PSDU.

The A-MPDU subframe includes an MPDU delimiter, and an MPDU may beoptionally included after the MPDU delimiter. Furthermore, a pad octetis attached to the MPDU in order to make the length of each A-MPDUsubframe in a multiple of 4 octets other than the last A-MPDU subframewithin one A-MPDU.

The MPDU delimiter includes a reserved field, an MPDU length field, acyclic redundancy check (CRC) field, and a delimiter signature field.

In the case of a VHT PPDU, the MPDU delimiter may further include anend-of-frame (EOF) field. If an MPDU length field is 0 and an A-MPDUsubframe or A-MPDU used for padding includes only one MPDU, in the caseof an A-MPDU subframe on which a corresponding MPDU is carried, the EOFfield is set to “1.” If not, the EOF field is set to The MPDU lengthfield includes information about the length of the MPDU.

If an MPDU is not present in a corresponding A-MPDU subframe, the PDUlength field is set to “0.” An A-MPDU subframe in which an MPDU lengthfield has a value of “0” is used to be padded to a corresponding A-MPDUin order to match the A-MPDU up with available octets within a VHT PPDU.

The CRC field includes CRC information for an error check. The delimitersignature field includes pattern information used to search for an MPDUdelimiter.

Furthermore, the MPDU includes an MAC header, a frame body, and a framecheck sequence (FCS).

FIG. 18 is a diagram illustrating a DL multi-user (MU) PPDU format in awireless communication system to which an embodiment of the presentinvention may be applied.

In FIG. 18, the number of STAs receiving a corresponding PPDU is assumedto be 3 and the number of spatial streams allocated to each STA isassumed to be 1, but the number of STAs paired with an AP and the numberof spatial streams allocated to each STA are not limited thereto.

Referring to FIG. 18, the MU PPDU is configured to include L-TFs (i.e.,an L-STF and an L-LTF), an L-SIG field, a VHT-SIG-A field, a VHT-TFs(i.e., a VHT-STF and a VHT-LTF), a VHT-SIG-B field, a service field, oneor more PSDUs, a padding field, and a tail bit. The L-TFs, the L-SIGfield, the VHT-SIG-A field, the VHT-TFs, and the VHT-SIG-B field are thesame as those of FIG. 4, and a detailed description thereof is omitted.

Information for indicating PPDU duration may be included in the L-SIGfield. In the PPDU, PPDU duration indicated by the L-SIG field includesa symbol to which the VHT-SIG-A field has been allocated, a symbol towhich the VHT-TFs have been allocated, a field to which the VHT-SIG-Bfield has been allocated, bits forming the service field, bits forming aPSDU, bits forming the padding field, and bits forming the tail field.An STA receiving the PPDU may obtain information about the duration ofthe PPDU through information indicating the duration of the PPDUincluded in the L-SIG field.

As described above, group ID information and time and spatial streamnumber information for each user are transmitted through the VHT-SIG-A,and a coding method and MCS information are transmitted through theVHT-SIG-B. Accordingly, beamformees may check the VHT-SIG-A and theVHT-SIG-B and may be aware whether a frame is an MU MIMO frame to whichthe beamformee belongs. Accordingly, an STA which is not a member STA ofa corresponding group ID or which is a member of a corresponding groupID, but in which the number of streams allocated to the STA is “0” isconfigured to stop the reception of the physical layer to the end of thePPDU from the VHT-SIG-A field, thereby being capable of reducing powerconsumption.

In the group ID, an STA can be aware that a beamformee belongs to whichMU group and it is a user who belongs to the users of a group to whichthe STA belongs and who is placed at what place, that is, that a PPDU isreceived through which stream by previously receiving a group IDmanagement frame transmitted by a beamformer.

All of MPDUs transmitted within the VHT MU PPDU based on 802.11ac areincluded in the A-MPDU. In the data field of FIG. 18, each VHT A-MPDUmay be transmitted in a different stream.

In FIG. 18, the A-MPDUs may have different bit sizes because the size ofdata transmitted to each STA may be different.

In this case, null padding may be performed so that the time when thetransmission of a plurality of data frames transmitted by a beamformeris ended is the same as the time when the transmission of a maximuminterval transmission data frame is ended. The maximum intervaltransmission data frame may be a frame in which valid downlink data istransmitted by a beamformer for the longest time. The valid downlinkdata may be downlink data that has not been null padded. For example,the valid downlink data may be included in the A-MPDU and transmitted.Null padding may be performed on the remaining data frames other thanthe maximum interval transmission data frame of the plurality of dataframes.

For the null padding, a beamformer may fill one or more A-MPDUsubframes, temporally placed in the latter part of a plurality of A-MPDUsubframes within an A-MPDU frame, with only an MPDU delimiter fieldthrough encoding. An A-MPDU subframe having an MPDU length of 0 may becalled a null subframe.

As described above, in the null subframe, the EOF field of the MPDUdelimiter is set to “1.” Accordingly, when the EOF field set to 1 isdetected in the MAC layer of an STA on the receiving side, the receptionof the physical layer is stopped, thereby being capable of reducingpower consumption.

Block Ack Procedure

FIG. 19 is a diagram illustrating a downlink MU-MIMO transmissionprocess in a wireless communication system to which the presentinvention may be applied.

MI-MIMO in 802.11ac works only in the downlink direction from the AP toclients. A multi-user frame can be transmitted to multiple receivers atthe same time, but the acknowledgements must be transmitted individuallyin the uplink direction.

Every MPDU transmitted in a VHT MU PPDU based on 802.11ac is included inan A-MPDU, so responses to A-MPDUs within the VHT MU PPDU that are notimmediate responses to the VHT MU PPDU are transmitted in response toBAR (Block Ack Request) frames by the AP.

To begin with, the AP transmits a VHT MU PPDU (i.e., a preamble anddata) to every receiver (i.e., STA 1, STA 2, and STA 3). The VHT MU PPDUincludes VHT A-MPDUs that are to be transmitted to each STA.

Having received the VHT MU PPDU from the AP, STA 1 transmits a BA (BlockAcknowledgement) frame to the AP after an SIFS. A more detaileddescription of the BA frame will be described later.

Having received the BA from STA 1, the AP transmits a BAR (blockacknowledgement request) frame to STA 2 after an SIFS, and STA 2transmits a BA frame to the AP after an SIFS. Having received the BAframe from STA 2, the AP transmits a BAR frame to STA 3 after an SIFS,and STA 3 transmits a BA frame to the AP after an SIFS.

When this process is performed all STAs, the AP transmits the next MUPPDU to all the STAs.

ACK (Acknowledgement)/Block ACK Frames

In general, an ACK frame is used as a response to an MPDU, and a blockACK frame is used as a response to an A-MPDU.

FIG. 20 is a diagram illustrating an ACK frame in a wirelesscommunication system to which the present invention may be applied.

Referring to FIG. 20, the ACK frame consists of a Frame Control field, aDuration field, an RA field, and a FCS.

The RA field is set to the value of the Address 2 field of theimmediately preceding Data frame, Management frame, Block Ack Requestframe, Block Ack frame, or PS-Poll frame.

For ACK frames sent by non-QoS STAs, if the More Fragments subfield isset to 0 in the Frame Control field of the immediately preceding Data orManagement frame, the duration value is set to 0.

For ACK frames not sent by non-QoS STAs, the duration value is set tothe value obtained from the Duration/ID field of the immediatelypreceding Data, Management, PS-Poll, BlockAckReq, or BlockAck frameminus the time, in microseconds, required to transmit the ACK frame andits SIFS interval. If the calculated duration includes a fractionalmicrosecond, that value is rounded up to the next higher integer.

Hereinafter, the Block Ack Request frame will be discussed.

FIG. 21 is a diagram illustrating a Block Ack Request frame in awireless communication system to which the present invention may beapplied.

Referring to FIG. 21, the Block Ack Request frame consists of a FrameControl field, a Duration/ID field, an RA field, a TA field, a BARControl field, a BAR Information field, and a frame check sequence(FCS).

The RA field may be set to the address of the STA receiving the BARframe.

The TA field may be set to the address of the STA transmitting the BARframe.

The BAR Control field includes a BAR Ack Policy subfield, a Multi-TIDsubfield, a Compressed Bitmap subfield, a Reserved subfield, and a TIDInfo subfield.

Table 10 shows the BAR Control field.

TABLE 10 Subfield Bits Description BAR Ack 1 Set to 0 when the senderrequires immediate ACK of a data Policy transmission. Set to 1 when thesender does not require immediate ACK of a data transmission. Multi-TID1 Indicates the type of the BAR frame depending on the values of theCompressed 1 Multi-TID subfield and Compressed Bitmap subfield. Bitmap00: Basic BAR 01: Compressed BAR 10: Reserved 11: Multi-TID BAR Reserved9 TID_Info 4 The meaning of the TID_Info field depends on the type ofthe BAR frame. For a Basic BAR frame and a Compressed BAR frame, thissubfield contains information on TIDs for which a BA frame is required.For a Multi-TID BAR frame, this subfield contains the number of TIDs.

The BAR Information field contains different information depending onthe type of the BAR frame. This will be described with reference to FIG.22.

FIG. 22 is a diagram illustrating the BAR Information field of a BlockAck Request frame in a wireless communication system to which thepresent invention may be applied.

(a) of FIG. 22 illustrates the BAR Information field of Basic BAR andCompressed BAR frames, and (b) of FIG. 22 illustrates the BARInformation field of a Multi-TID BAR frame.

Referring to (a) of FIG. 22, for the Basic BAR and Compressed BARframes, the BAR Information field includes a Block Ack Starting SequenceControl subfield.

The Block Ack Starting Sequence Control subfield includes a FragmentNumber subfield and a Starting Sequence Number subfield.

The Fragment Number subfield is set to 0.

For the Basic BAR frame, the Starting Sequence Number subfield containsthe sequence number of the first MSDU for which the corresponding BARframe is sent. For the Compressed BAR frame, the Starting SequenceControl subfield contains the sequence number of the first MSDU orA-MSDU for which the corresponding BAR frame is sent.

Referring to (b) of FIG. 22, for the Multi-TID BAR frame, the BARInformation field includes a Per TID Info subfield and a Block AckStarting Sequence Control subfield, which are repeated for each TID.

The Per TID Info subfield includes a Reserved subfield and a TID Valuesubfield. The TID Value subfield contains a TID value.

As described above, the Block Ack Starting Sequence Control subfieldincludes fragment Number and Starting Sequence Number subfields. TheFragment Number subfield is set to 0. The Starting Sequence Controlsubfield contains the sequence number of the first MSDU or A-MSDU forwhich the corresponding BAR frame is sent.

FIG. 23 is a diagram illustrating a Block Ack frame in a wirelesscommunication system to which the present invention may be applied.

Referring to FIG. 23, the Block Ack (BA) frame consists of a FrameControl field, a Duration/ID field, an RA field, a TA field, a BAControl field, a BA Information field, and a Frame Check Sequence (FCS).

The RA field may be set to the address of the STA requesting the BAframe.

The TA field may be set to the address of the STA transmitting the BAframe.

The BA Control field includes a BA Ack Policy subfield, a Multi-TIDsubfield, a Compressed Bitmap subfield, a Reserved subfield, and a TIDInfo subfield.

Table 11 shows the BA Control field.

TABLE 11 Subfield Bits Description BA Ack Policy 1 Set to 0 when thesender requires immediate ACK of a data transmission. Set to 1 when thesender does not require immediate ACK of a data transmission. Multi-TID1 Indicates the type of the BA frame depending on the values of theCompressed 1 Multi-TID subfield and Compressed Bitmap subfield. Bitmap00: Basic BA 01: Compressed BA 10: Reserved 11: Multi-TID BA Reserved 9TID_Info 4 The meaning of the TID_Info field depends on the type of theBA frame. For a Basic BA frame and a Compressed BA frame, this subfieldcontains information on TIDs for which a BA frame is required. For aMulti-TID BA frame, this subfield contains the number of TIDs.

The BA Information field contains different information depending on thetype of the BA frame. This will be described with reference to FIG. 24.

FIG. 24 is a diagram illustrating the BA Information field of a BlockAck frame in a wireless communication system to which the presentinvention may be applied.

(a) of FIG. 24 illustrates the BA Information field of a Basic BA frame,(b) of FIG. 24 illustrates the BA Information field of a Compressed BARframe, and (c) of FIG. 24 illustrates the BA Information field of aMulti-TID BA frame.

Referring to (a) of FIG. 24, for the Basic BA frame, the BA Informationfield includes a Block Ack Starting Sequence Control subfield and aBlock Ack Bitmap subfield.

As described above, the Block Ack Starting Sequence Control subfieldincludes a Fragment Number subfield and a Starting Sequence Numbersubfield.

The Fragment Number subfield is set to 0.

The Starting Sequence Number subfield contains the sequence number ofthe first MSDU for which the corresponding BA frame is sent, and is setto the same value as the immediately preceding Basic BAR frame.

The Block Ack Bitmap subfield is 128 octets in length and is used toindicate the received status of a maximum of 64 MSDUs. If a bit of theBlock Ack Bitmap subfield has a value of ‘1’, it indicates thesuccessful reception of a single MSDU corresponding to that bitposition, and if a bit of the Block Ack Bitmap subfield has a value of‘0’, it indicates the unsuccessful reception of a single MSDUcorresponding to that bit position.

Referring to (b) of FIG. 24, for the Compressed BA frame, the BAInformation field includes a Block Ack Starting Sequence Controlsubfield and a Block Ack Bitmap subfield.

As described above, the Block Ack Starting Sequence Control subfieldincludes a Fragment Number subfield and a Starting Sequence Numbersubfield.

The Fragment Number subfield is set to 0.

The Starting Sequence Number subfield contains the sequence number ofthe first MSDU or A-MSDU for which the corresponding BA frame is sent,and is set to the same value as the immediately preceding Basic BARframe.

The Block Ack Bitmap subfield is 8 octets in length and is used toindicate the received status of a maximum of 64 MSDUs and A-MSDU. If abit of the Block Ack Bitmap subfield has a value of ‘1’, it indicatesthe successful reception of a single MSDU or A-MSDU corresponding tothat bit position, and if a bit of the Block Ack Bitmap subfield has avalue of ‘0’, it indicates the unsuccessful reception of a single MSDUor A-MSDU corresponding to that bit position.

Referring to (c) of FIG. 24, for the Multi-TID BA frame, the BAInformation field includes a Per TID Info subfield and a Block AckStarting Sequence Control subfield, which are repeated for each TID inorder of increasing TID.

The Per TID Info subfield includes a Reserved subfield and a TID Valuesubfield. The TID Value subfield contains a TID value.

As described above, the Block Ack Starting Sequence Control subfieldincludes fragment Number and Starting Sequence Number subfields. TheFragment Number subfield is set to 0. The Starting Sequence Controlsubfield contains the sequence number of the first MSDU or A-MSDU forwhich the corresponding BA frame is sent.

The Block Ack Bitmap subfield is 8 octets in length. If a bit of theBlock Ack Bitmap subfield has a value of ‘1’, it indicates thesuccessful reception of a single MSDU or A-MSDU corresponding to thatbit position, and if a bit of the Block Ack Bitmap subfield has a valueof ‘0’, it indicates the unsuccessful reception of a single MSDU orA-MSDU corresponding to that bit position.

UL Multiple User (MU) Transmission Method

A new frame format and numerology for an 802.11ax system, that is, thenext-generation WLAN system, are actively discussed in the situation inwhich vendors of various fields have lots of interests in thenext-generation Wi-Fi and a demand for high throughput and quality ofexperience (QoE) performance improvement are increased after 802.11ac.

IEEE 802.11ax is one of WLAN systems recently and newly proposed as thenext-generation WLAN systems for supporting a higher data rate andprocessing a higher user load, and is also called a so-called highefficiency WLAN (HEW).

An IEEE 802.11ax WLAN system may operate in a 2.4 GHz frequency band anda 5 GHz frequency band like the existing WLAN systems. Furthermore, theIEEE 802.11ax WLAN system may also operate in a higher 60 GHz frequencyband.

In the IEEE 802.11ax system, an FFT size four times larger than that ofthe existing IEEE 802.11 OFDM systems (e.g., IEEE 802.11a, 802.11n, and802.11ac) may be used in each bandwidth for average throughputenhancement and outdoor robust transmission for inter-symbolinterference. This is described below with reference to relateddrawings.

Hereinafter, in a description of an HE format PPDU according to anembodiment of the present invention, the descriptions of theaforementioned non-HT format PPDU, HT mixed format PPDU, HT-green fieldformat PPDU and/or VHT format PPDU may be reflected into the descriptionof the HE format PPDU although they are not described otherwise.

FIG. 25 is a diagram illustrating a high efficiency (HE) format PPDUaccording to an embodiment of the present invention.

FIG. 25(a) illustrates a schematic configuration of the HE format PPDU,and FIGS. 25(b) to 25(d) illustrate more detailed configurations of theHE format PPDU.

Referring to FIG. 25(a), the HE format PPDU for an HEW may basicallyinclude a legacy part (L-part), an HE-part, and an HE-data field.

The L-part includes an L-STF, an L-LTF, and an L-SIG field as in a formmaintained in the existing WLAN system. The L-STF, the L-LTF, and theL-SIG field may be called a legacy preamble.

The HE-part is a part newly defined for the 802.11ax standard and mayinclude an HE-STF, a HE-SIG field, and an HE-LTF. In FIG. 25(a), thesequence of the HE-STF, the HE-SIG field, and the HE-LTF is illustrated,but the HE-STF, the HE-SIG field, and the HE-LTF may be configured in adifferent sequence. Furthermore, the HE-LTF may be omitted. Not only theHE-STF and the HE-LTF, but the HE-SIG field may be commonly called anHE-preamble.

The HE-SIG may include information (e.g., OFDMA, UL MU MIMO, andimproved MCS) for decoding the HE-data field.

The L-part and the HE-part may have different fast Fourier transform(FFT) sizes (i.e., different subcarrier spacing) and use differentcyclic prefixes (CPs).

In an 802.11ax system, an FFT size four times (4×) larger than that of alegacy WLAN system may be used. That is, the L-part may have a 1× symbolstructure, and the HE-part (more specifically, HE-preamble and HE-data)may have a 4× symbol structure. In this case, the FFT of a 1×, 2×, or 4×size means a relative size for a legacy WLAN system (e.g., IEEE 802.11a,802.11n, and 802.11ac).

For example, if the sizes of FFTs used in the L-part are 64, 128, 256,and 512 in 20 MHz, 40 MHz, 80 MHz, and 160 MHz, respectively, the sizesof FFTs used in the HE-part may be 256, 512, 1024, and 2048 in 20 MHz,40 MHz, 80 MHz, and 160 MHz, respectively.

If an FFT size is larger than that of a legacy WLAN system as describedabove, subcarrier frequency spacing is reduced. Accordingly, the numberof subcarriers per unit frequency is increased, but the length of anOFDM symbol is increased.

That is, if a larger FFT size is used, it means that subcarrier spacingis narrowed. Likewise, it means that an inverse discrete Fouriertransform (IDFT)/discrete Fourier transform (DFT) period is increased.In this case, the IDFT/DFT period may mean a symbol length other than aguard interval (GI) in an OFDM symbol.

Accordingly, if an FFT size four times larger than that of the L-part isused in the HE-part (more specifically, the HE-preamble and the HE-datafield), the subcarrier spacing of the HE-part becomes 1/4 times thesubcarrier spacing of the L-part, and the IDFT/DFT period of the HE-partis four times the IDFT/DFT period of the L-part. For example, if thesubcarrier spacing of the L-part is 312.5 kHz (=20 MHz/64, 40 MHz/128,80 MHz/256 and/or 160 MHz/512), the subcarrier spacing of the HE-partmay be 78.125 kHz (=20 MHz/256, 40 MHz/512, 80 MHz/1024 and/or 160MHz/2048). Furthermore, if the IDFT/DFT period of the L-part is 3.2 μs(=1/312.5 kHz), the IDFT/DFT period of the HE-part may be 12.8 μs(=1/78.125 kHz).

In this case, since one of 0.8 μs, 1.6 μs, and 3.2 μs may be used as aGI, the OFDM symbol length (or symbol interval) of the HE-part includingthe GI may be 13.6 μs, 14.4 μs, or 16 μs depending on the GI.

Referring to FIG. 25 (b), the HE-SIG field may be divided into aHE-SIG-A field and a HE-SIG-B field.

For example, the HE-part of the HE format PPDU may include a HE-SIG-Afield having a length of 12.8 μs, an HE-STF of 1 OFDM symbol, one ormore HE-LTFs, and a HE-SIG-B field of 1 OFDM symbol.

Furthermore, in the HE-part, an FFT size four times larger than that ofthe existing PPDU may be applied from the HE-STF other than the HE-SIG-Afield. That is, FFTs having 256, 512, 1024, and 2048 sizes may beapplied from the HE-STFs of the HE format PPDUs of 20 MHz, 40 MHz, 80MHz, and 160 MHz, respectively.

In this case, if the HE-SIG field is divided into the HE-SIG-A field andthe HE-SIG-B field as in FIG. 25(b), the positions of the HE-SIG-A fieldand the HE-SIG-B field may be different from those of FIG. 25(b). Forexample, the HE-SIG-B field may be transmitted after the HE-SIG-A field,and the HE-STF and the HE-LTF may be transmitted after the HE-SIG-Bfield. In this case, an FFT size four times larger than that of theexisting PPDU may be applied from the HE-STF.

Referring to FIG. 25(c), the HE-SIG field may not be divided into aHE-SIG-A field and a HE-SIG-B field.

For example, the HE-part of the HE format PPDU may include an HE-STF of1 OFDM symbol, a HE-SIG field of 1 OFDM symbol, and one or more HE-LTFs.

In the manner similar to that described above, an FFT size four timeslarger than that of the existing PPDU may be applied to the HE-part.That is, FFT sizes of 256, 512, 1024, and 2048 may be applied from theHE-STF of the HE format PPDU of 20 MHz, 40 MHz, 80 MHz, and 160 MHz,respectively.

Referring to FIG. 25(d), the HE-SIG field is not divided into a HE-SIG-Afield and a HE-SIG-B field, and the HE-LTF may be omitted.

For example, the HE-part of the HE format PPDU may include an HE-STF of1 OFDM symbol and a HE-SIG field of 1 OFDM symbol.

In the manner similar to that described above, an FFT size four timeslarger than that of the existing PPDU may be applied to the HE-part.That is, FFT sizes of 256, 512, 1024, and 2048 may be applied from theHE-STF of the HE format PPDU of 20 MHz, 40 MHz, 80 MHz, and 160 MHz,respectively.

The HE format PPDU for the WLAN system to which the present inventionmay be applied may be transmitted through at least one 20 MHz channel.For example, the HE format PPDU may be transmitted in the 40 MHz, 80 MHzor 160 MHz frequency band through total four 20 MHz channel. This willbe described in more detail with reference to the drawing below.

Hereinafter, the PPDU format is described based on FIG. 25(b) above, forthe convenience of description, but the present invention is not limitedthereto.

FIG. 26 is a diagram illustrating a HE format PPDU according to anembodiment of the present invention.

FIG. 26 illustrates a PPDU format when 80 MHz is allocated to one STA(or OFDMA resource units are allocated to multiple STAs within 80 MHz)or when different streams of 80 MHz are allocated to multiple STAs,respectively.

Referring to FIG. 26, an L-STF, an L-LTF, and an L-SIG may betransmitted an OFDM symbol generated on the basis of 64 FFT points (or64 subcarriers) in each 20 MHz channel.

A HE-SIG-A field may include common control information commonlyreceived by STAs which receive a PPDU. The HE-SIG-A field may betransmitted in 1 to 3 OFDM symbols. The HE-SIG-A field is duplicated foreach 20 MHz and contains the same information. Also, the HE-SIG-A fieldindicates full bandwidth information of the system.

Table 12 illustrates information contained in the HE-SIG-A field.

TABLE 12 Field Bits Description Bandwidth 2 Indicates a bandwidth inwhich a PPDU is transmitted. For example, 20 MHz, 40 MHz, 80 MHz or 160MHz Group ID 6 Indicates an STA or a group of STAs that will receive aPPDU Stream 12 Indicates the number or location of spatial streams foreach STA or information the number or location of spatial streams for agroup of STAs UL indication 1 Indicates whether a PPDU is destined to anAP (uplink) or STA (downlink) MU indication 1 Indicates whether a PPDUis an SU-MIMO PPDU or an MU-MIMO PPDU GI indication 1 Indicates whethera short GI or a long GI is used Allocation 12 Indicates a band or achannel (subchannel index or subband index) information allocated toeach STA in a bandwidth in which a PPDU is transmitted Transmission 12Indicates a transmission power for each channel or each STA power

Information contained in each of the fields illustrated in Table 12 maybe as defined in the IEEE 802.11 system. Also, the above-describedfields are examples of the fields that may be included in the PPDU butnot limited to them. That is, the above-described fields may besubstituted with other fields or further include additional fields, andnot all of the fields may be necessarily included.

The HE-STF field is used to improve AGC estimation in MIMO transmission.

The HE-SIG-B field may include user-specific information that isrequired for each STA to receive its own data (i.e., a Physical LayerService Data Unit (PSDU)). The HE-SIG-B field may be transmitted in oneor two OFDM symbols. For example, the HE-SIG-B field may includeinformation about the length of a corresponding PSDU and the Modulationand Coding Scheme (MCS) of the corresponding PSDU.

The L-STF field, the L-LTF field, the L-SIG field, and the HE-SIG-Afield may be duplicately transmitted every 20 MHz channel. For example,when a PPDU is transmitted through four 20 MHz channels, the L-STFfield, the L-LTF field, L-STG field, and the HE-SIG-A field may beduplicately transmitted every 20 MHz channel.

If the FFT size is increased, a legacy STA that supports conventionalIEEE 802.11a/g/n/ac may be unable to decode a corresponding PPDU. Forcoexistence between a legacy STA and a HE STA, the L-STF, L-LTF, andL-SIG fields are transmitted through 64 FFT in a 20 MHz channel so thatthey can be received by a legacy STA. For example, the L-SIG field mayoccupy a single OFDM symbol, a single OFDM symbol time may be 4 μs, anda GI may be 0.8 μs.

An FFT size per unit frequency may be further increased from the HE-STF(or from the HE-SIG-A). For example, 256 FFT may be used in a 20 MHzchannel, 512 FFT may be used in a 40 MHz channel, and 1024 FFT may beused in an 80 MHz channel. If the FFT size is increased, the number ofOFDM subcarriers per unit frequency is increased because spacing betweenOFDM subcarriers is reduced, but an OFDM symbol time may be increased.In order to improve system efficiency, the length of a GI after theHE-STF may be set equal to the length of the GI of the HE-SIG-A.

The HE-SIG-A field includes information that is required for a HE STA todecode a HE PPDU. However, the HE-SIG-A field may be transmitted through64 FFT in a 20 MHz channel so that it may be received by both a legacySTA and a HE STA. The reason for this is that a HE STA is capable ofreceiving conventional HT/VHT format PPDUs in addition to a HE formatPPDU. In this case, it is required that a legacy STA and a HE STAdistinguish a HE format PPDU from an HT/VHT format PPDU, and vice versa.

FIG. 27 is a diagram illustrating a HE format PPDU according to anembodiment of the present invention.

Referring to FIG. 27, it is identical to that illustrated in FIG. 26above, except that the HE-SIG-B field comes next to the HE-SIG-A field.In this case, an FFT size per unit frequency may be further increasedfrom the HE-STF (or from the HE-SIG-B). For example, from the HE-STF (orfrom the HE-SIG-B), 256 FFT may be used in a 20 MHz channel, 512 FFT maybe used in a 40 MHz channel, and 1024 FFT may be used in an 80 MHzchannel.

FIG. 28 is a diagram illustrating a HE format PPDU according to anembodiment of the present invention.

In FIG. 28, it is assumed that 20 MHz channels are allocated todifferent STAs (e.g., STA 1, STA 2, STA 3, and STA 4).

Referring to FIG. 28, the HE-SIG B field is located next to the HE-SIG Afield. In this case, an FFT size per unit frequency may be moreincreased from the HE-STF (or HE-SIG-B). For example, 256 FFT from theHE-STF (or HE-SIG-B) may be used in the 20 MHz channel, 512 FFT may beused in the 40 MHz channel, and 1024 FFT may be used in the 80 MHzchannel.

Information transmitted in each of the fields in a PPDU is the same asillustrated in FIG. 26 above, so its description will be omitted.

The HE-SIG-B field contains information specific to each STA, but isencoded over the entire band (that is, indicated by the HE-SIG-A field).That is, the HE-SIG-B field contains information on all STAs and isreceived by all the STAs.

The HE-SIG-B field may indicate information on a frequency bandwidthallocated to each STA and/or stream information for the correspondingfrequency bandwidth. For example, in the HE-SIG-B of FIG. 28, a first 20MHz bandwidth may be allocated to STA 1, a second 20 MHz bandwidth maybe allocated to STA 2, a third 20 MHz bandwidth may be allocated to STA3, and a fourth 20 MHz bandwidth may be allocated to STA 4. Also, afirst 40 MHz bandwidth may be allocated to STA 1 and STA 2, and a second40 MHz bandwidth may be allocated to STA 3 and STA 4. In this case,different streams may be allocated to STA 1 and STA 2, and differentstreams may be allocated to STA 3 and STA 4.

Moreover, a HE-SIG C field may be defined and added to what isillustrated in FIG. 28. In this case, in the HE-SIG-B field, informationon all STAs is transmitted over the entire bandwidth, and controlinformation specific to each STA may be transmitted every 20 MHz.

In addition, same as the FIG. 26 to FIG. 28, the HE-SIG-B field may notbe transmitted throughout the entire band, but may be transmitted in aunit of 20 MHz as the same as the HE-SIG-A field. This will be describedin more detail with reference to the drawings below.

FIG. 29 is a diagram exemplifying an HE format PPDU according to anembodiment of the present invention.

In FIG. 29, a case is assumed that 20 MHz channels are allocated todifferent STAs (e.g., STA 1, STA 2, STA 3 and STA4).

Referring to FIG. 29, like the case of FIG. 28, the HE-SIG B field islocated next to the HE-SIG A field. However, the HE-SIG B field may notbe transmitted throughout the entire band, but may be transmitted in aunit of 20 MHz as the same as the HE-SIG-A field.

In this case, an FFT size per unit frequency may be more increased fromthe HE-STF (or HE-SIG-B). For example, 256 FFT from the HE-STF (orHE-SIG-B) may be used in the 20 MHz channel, 512 FFT may be used in the40 MHz channel, and 1024 FFT may be used in the 80 MHz channel.

Since the information transmitted in each field included in a PPDU isthe same as the example of FIG. 26, the description for it will beomitted below.

The HE-SIG A field is transmitted with being duplicated in a unit of 20MHz.

The HE-SIG B field may indicate the frequency bandwidth informationallocated to each STA and/or the stream information in the correspondingfrequency band.

The HE-SIG B field may be transmitted in a unit of 20 MHz as the same asthe HE-SIG A field. Since the HE-SIG B field includes the information ofeach STA, the information of each STA may be included in each HE-SIG Bfield in a unit of 20 MHz. In this case, although the example of FIG. 29shows the case that 20 MHz is allocated to each STA, in the case that 40MHz is allocated to an STA, the HE-SIG B field may be transmitted withbeing duplicated in a unit of 20 MHz.

In addition, the information of all STAs is included in the HE-SIG Bfield (i.e., the information specified to each STA is merged) and theHE-SIG B field may be transmitted with being duplicated in a unit of 20MHz as the same as the HE-SIG A field.

As exemplified in FIG. 27 to FIG. 29, in the case that the HE-SIG Bfield is located in front of the HE STF field and the HE-LTF field, theHE-SIG B field is configured with a short symbol length using 64 FFTs in20 MHz band. As exemplified in FIG. 26, in the case that the HE-SIG Bfield is located behind of the HE STF field and the HE-LTF field, theHE-SIG B field is configured with a long symbol length using 256 FFTs in20 MHz band.

In the case that a part of the bandwidth of which interference levelfrom an adjacent BSS is small in the situation that different bandwidthis supported for each BSS, it may be more preferable that the HE-SIG Bfield is not transmitted through the entire band as described above.

In FIG. 26 to FIG. 29, the data field is a payload, and may include aSERVICE field, a scrambled PSDU, tail bits and padding bits.

FIG. 30 illustrates an example of phase rotation for classification ofHE format PPDUs.

For classification of HE format PPDUs, the phases of 3 OFDM symbolstransmitted after the L-SIG field may be used in a HE format PPDU.

Referring to FIG. 30, the phases of the OFDM symbol #1 and the OFDMsymbol #2 are not rotated, but the phase of the OFDM symbol #3 isrotated counterclockwise by 90 degrees. That is, the OFDM symbols #1 and#2 is modulated by BPSK, and the OFDM symbol #3 is modulated by QBPSK.

An STA attempts to decode the first to third OFDM symbols transmittedafter the L-SIG field of the received PDU, based on the constellationsillustrated in (b) of FIG. 30. If the STA succeeds in decoding, thecorresponding PPDU may be classified as an HT format PPDU.

Here, if the HE-SIG-A field is transmitted in 3 OFDM symbols after theL-SIG field, it may be said that all the OFDM symbols #1 to #3 are usedto send the HE-SIG-A field.

A multi-user UL transmission method in a WLAN system is described below.

A method of transmitting, by an AP operating in a WLAN system, data to aplurality of STAs on the same time resource may be called downlinkmulti-user (DL MU) transmission. In contrast, a method of transmitting,by a plurality of STAs operating in a WLAN system, data to an AP on thesame time resource may be called uplink multi-user (UL MU) transmission.

Such DL MU transmission or UL MU transmission may be multiplexed on afrequency domain or a space domain.

If DL MU transmission or UL MU transmission is multiplexed on thefrequency domain, different frequency resources (e.g., subcarriers ortones) may be allocated to each of a plurality of STAs as DL or ULresources based on orthogonal frequency division multiplexing (OFDMA). Atransmission method through different frequency resources in such thesame time resources may be called “DL/UL MU OFDMA transmission.”

If DL MU transmission or UL MU transmission is multiplexed on the spacedomain, different spatial streams may be allocated to each of aplurality of STAs as DL or UL resources. A transmission method throughdifferent spatial streams on such the same time resources may be called“DL/UL MU MIMO transmission.”

Current WLAN systems do not support UL MU transmission due to thefollowing constraints.

Current WLAN systems do not support synchronization for the transmissiontiming of UL data transmitted by a plurality of STAs. For example,assuming that a plurality of STAs transmits UL data through the sametime resources in the existing WLAN system, in the present WLAN systems,each of a plurality of STAs is unaware of the transmission timing of ULdata of another STA. Accordingly, an AP may not receive UL data fromeach of a plurality of STAs on the same time resource.

Furthermore, in the present WLAN systems, overlap may occur betweenfrequency resources used by a plurality of STAs in order to transmit ULdata. For example, if a plurality of STAs has different oscillators,frequency offsets may be different. If a plurality of STAs havingdifferent frequency offsets performs UL transmission at the same timethrough different frequency resources, frequency regions used by aplurality of STAs may partially overlap.

Furthermore, in existing WLAN systems, power control is not performed oneach of a plurality of STAs. An AP dependent on the distance betweeneach of a plurality of STAs and the AP and a channel environment mayreceive signals of different power from a plurality of STAs. In thiscase, a signal having weak power may not be relatively detected by theAP compared to a signal having strong power.

Accordingly, an embodiment of the present invention proposes an UL MUtransmission method in a WLAN system.

FIG. 31 is a diagram illustrating an uplink multi-user transmissionprocedure according to an embodiment of the present invention.

Referring to FIG. 31, an AP may instruct STAs participating in UL MUtransmission to prepare for UL MU transmission, receive an UL MU dataframe from these STAs, and send an ACK frame (BA (Block Ack) frame) inresponse to the UL MU data frame.

First of all, the AP instructs STAs that will transmit UL MU data toprepare for UL MU transmission by sending an UL MU Trigger frame 3110.Here, the term UL MU scheduling frame may be called “UL MU schedulingframe”.

Here, the UL MU Trigger frame 3110 may contain control information suchas STA ID (identifier)/address information, information on theallocation of resources to be used by each STA, and durationinformation.

The STA ID/address information refers to information on the identifieror address for specifying an STA that transmits uplink data.

The resource allocation information refers to information on uplinktransmission resources allocated to each STA (e.g., information onfrequency/subcarriers allocated to each STA in the case of UL MU OFDMAtransmission and a stream index allocated to each STA in the case of ULMU MIMO transmission).

The duration information refers to information for determining timeresources for transmitting an uplink data frame sent by each of multipleSTAs.

For example, the duration information may include period information ofa TXOP (Transmit Opportunity) allocated for uplink transmission of eachSTA or information (e.g., bits or symbols) on the uplink frame length.

Also, the UL MU Trigger frame 3110 may further include controlinformation such as information on an MCS to be used when each STA sendsan UL MU data frame, coding information, etc.

The above-mentioned control information may be transmitted in a HE-part(e.g., the HE-SIG-A field or HE-SIG-B field) of a PPDU for deliveringthe UL MU Trigger frame 3110 or in the control field of the UL MUTrigger frame 3110 (e.g., the Frame Control field of the MAC frame).

The PPDU for delivering the UL MU Trigger frame 3110 starts with anL-part (e.g., the L-STF field, L-LTF field, and L-SIG field).Accordingly, legacy STAs may set their NAV (Network Allocation Vector)by L-SIG protection through the L-SIG field. For example, in the L-SIG,legacy STAs may calculate a period for NAV setting (hereinafter, ‘L-SIGprotection period’) based on the data length and data rate. The legacySTAs may determine that there is no data to be transmitted to themselvesduring the calculated L-SIG protection period.

For example, the L-SIG protection period may be determined as the sum ofthe value of the MAC Duration field of the UL MU Trigger frame 3110 andthe remaining portion after the L-SIG field of the PPDU delivering theUL MU Trigger frame 3110. Accordingly, the L-SIG protection period maybe set to a period of time until the transmission of an ACK frame 3130(or BA frame) transmitted to each STA, depending on the MAC durationvalue of the UL MU Trigger frame 3110.

Hereinafter, a method of resource allocation to each STA for UL MUtransmission will be described in more detail. A field containingcontrol information will be described separately for convenience ofexplanation, but the present invention is not limited to this.

A first field may indicate UL MU OFDMA transmission and UL MU MIMOtransmission in different ways. For example, ‘0’ may indicate UL MUOFDMA transmission, and ‘1’ may indicate UL MU MIMO transmission. Thefirst field may be 1 bit in size.

A second field (e.g., STA ID/address field) indicates the IDs oraddresses of STAs that will participate in UL MU transmission. The sizeof the second field may be obtained by multiplying the number of bitsfor indicating an STA ID by the number of STAs participating in UL MU.For example, if the second field has 12 bits, the ID/address of each STAmay be indicated in 4 bits.

A third field (e.g., resource allocation field) indicates a resourceregion allocated to each STA for UL MU transmission. Each STA may besequentially informed of the resource region allocated to it accordingto the order in the second field.

If the first field has a value of 0, this indicates frequencyinformation (e.g., frequency index, subcarrier index, etc.) for UL MUtransmission in the order of STA IDs/addresses in the second field, andif the first field has a value of 1, this indicates MIMO information(e.g., stream index, etc.) for UL MU transmission in the order of STAIDs/addresses in the second field.

In this case, a single STA may be informed of multiple indices (i.e.,frequency/subcarrier indices or stream indices). Thus, the third fieldmay be configured by multiplying the number of bits (or which may beconfigured in a bitmap format) by the number of STAs participating in ULMU transmission.

For example, it is assumed that the second field is set in the order ofSTA 1, STA 2, . . . , and the third field is set in the order of 2, 2, .. . .

In this case, if the first field is 0, frequency resources may beallocated to STA 1 and STA2, sequentially in the order of higherfrequency region (or lower frequency region). In an example, when 20 MHzOFDMA is supported in an 80 MHz band, STA 1 may use a higher (or lower)40 MHz band and STA 2 may use the subsequent 40 MHz band.

On the other hand, if the first field is 1, streams may be allocated toSTA 1 and STA 2, sequentially in the order of higher-order (orlower-order) streams. In this case, a beamforming scheme for each streammay be prescribed, or the third field or fourth field may contain morespecific information on the beamforming scheme for each stream.

Each STA sends a UL MU Data frame 3121, 3122, and 3123 to an AP based onthe UL MU Trigger frame 3110. That is, each STA may send a UL MU Dataframe 3121, 3122, and 3123 to an AP after receiving the UL MU Triggerframe 3110 from the AP.

Each STA may determine particular frequency resources for UL MU OFDMAtransmission or spatial streams for UL MU MIMO transmission, based onthe resource allocation information in the UL MU Trigger frame 3110.

Specifically, for UL MU OFDMA transmission, each STA may send an uplinkdata frame on the same time resource through a different frequencyresource.

Here, each of STA 1 to STA 3 may be allocated different frequencyresources for uplink data frame transmission, based on the STAID/address information and resource allocation information included inthe UL MU Trigger frame 3110. For example, the STA ID/addressinformation may sequentially indicate STA 1 to STA 3, and the resourceallocation information may sequentially indicate frequency resource 1,frequency resource 2, and frequency resource 3. In this case, STA 1 toSTA 3 sequentially indicated based on the STA ID/address information maybe allocated frequency resource 1, frequency resource 2, and frequencyresource 3, which are sequentially indicated based on the resourceallocation information. That is, STA 1, STA 2, and STA 3 may send theuplink data frame 3121, 3122, and 3123 to the AP through frequencyresource 1, frequency resource 2, and frequency resource 3,respectively.

For UL MU MIMO transmission, each STA may send an uplink data frame onthe same time resource through at least one different stream among aplurality of spatial streams.

Here, each of STA 1 to STA 3 may be allocated spatial streams for uplinkdata frame transmission, based on the STA ID/address information andresource allocation information included in the UL MU Trigger frame3110. For example, the STA ID/address information may sequentiallyindicate STA 1 to STA 3, and the resource allocation information maysequentially indicate spatial stream 1, spatial stream 2, and spatialstream 3. In this case, STA 1 to STA 3 sequentially indicated based onthe STA ID/address information may be allocated spatial stream 1,spatial stream 2, and spatial stream 3, which are sequentially indicatedbased on the resource allocation information. That is, STA 1, STA 2, andSTA 3 may send the uplink data frame 3121, 3122, and 3123 to the APthrough spatial stream 1, spatial stream 2, and spatial stream 3,respectively.

The PPDU for delivering the uplink data frame 3121, 3122, and 3123 mayhave a new structure, even without an L-part.

For UL MU MIMO transmission or for UL MU OFDMA transmission in a subbandbelow 20 MHz, the L-part of the PPDU for delivering the uplink dataframe 3121, 3122, and 3123 may be transmitted on an SFN (that is, allSTAs send an L-part having the same configuration and content). On thecontrary, for UL MU OFDMA transmission in a subband above 20 MHz, theL-part of the PPDU for delivering the uplink data frame 3121, 3122, and3123 may be transmitted every 20 MHz.

As long as the information in the UL MU Trigger frame 3110 suffices toconstruct an uplink data frame, the HE-SIG field (i.e., a part wherecontrol information for a data frame configuration scheme istransmitted) in the PPDU delivering the uplink data frame 3121, 3122,and 3123 may not be required. For example, the HE-SIG-A field and/or theHE-SIG-B field may not be transmitted. Also, the HE-SIG-A field and theHE-SIG C field may be transmitted, but the HE-SIG-B field may not betransmitted.

An AP may send an ACK Frame 3130 (or BA frame) in response to the uplinkdata frame 3121, 3122, and 3123 received from each STA. Here, the AP mayreceive the uplink data frame 3121, 3122, and 3123 from each STA andthen, after an SIFS, transmit the ACK frame 3130 to each STA.

Using the existing ACK frame structure, an RA field having a size of 6octets may include the AID (or Partial AID) of STAs participating in ULMU transmission.

Alternatively, an ACK frame with a new structure may be configured forDL SU transmission or DL MU transmission.

The AP may send an ACK frame 3130 to an STA only when an UL MU dataframe is successfully received by the corresponding STA. Through the ACKframe 3130, the AP may inform whether the reception is successful or notby ACK or NACK. If the ACK frame 3130 contains NACK information, it alsomay include the reason for NACK or information (e.g., UL MU schedulinginformation, etc.) for the subsequent procedure.

Alternatively, the PPDU for delivering the ACK frame 3130 may beconfigured to have a new structure without an L-part.

The ACK frame 3130 may contain STA ID or address information, but theSTA ID or address information may be omitted if the order of STAsindicated in the UL MU Trigger frame 3110 also applies to the ACK frame3130.

Moreover, the TXOP (i.e., L-SIG protection period) of the ACK frame 3130may be extended, and a frame for the next UL MU scheduling or a controlframe containing adjustment information for the next UL MU transmissionmay be included in the TXOP.

Meanwhile, an adjustment process may be added to synchronize STAs for ULMU transmission.

FIG. 32 is a diagram illustrating an uplink multi-user transmissionaccording to an embodiment of the present invention.

Hereinafter, description of the same parts as illustrated in FIG. 31above will be omitted for convenience of explanation.

Referring to FIG. 32, an AP may instruct STAs for use in UL MU toprepare for UL MU, and, after an adjustment process for synchronizationbetween the STAs for UL MU, receive an UL MU data frame and send an ACK.

First of all, the AP instructs STAs that will transmit UL MU data toprepare for UL MU transmission by sending an UL MU Trigger frame 3210.

Having received the UL MU Trigger frame 3210 from the AP, each STA sendsa Sync signal 3221, 3222, and 3223 to the AP. Here, each STA may receivethe UL MU Trigger frame 3210 and, after an SIFS, send the Sync signal3221, 3222, and 3233 to the AP.

Having received the Sync signal 3221, 3222, and 3223 from each STA, theAP sends an Adjustment frame 3230 to each STA. Here, the AP may receivethe Sync signal 3221, 3222, and 3233, and, after an SIFS, send theAdjustment frame 3230.

The procedure for sending and receiving the Sync signal 3221, 3222, and3223, and the Adjustment frame 3230 is a procedure for adjustingdifferences in timing/frequency/power among STAs for UL MU data frametransmission. That is, STAs send their Sync signal 3221, 3222, and 3233,and the AP informs each STA of adjustment information for adjustingdifferences in timing/frequency/power based on these values, through theAdjustment frame 3230 so that the STAs adjust and transmit these valuesin next UL MU data frame. Also, this procedure is performed after the ULMU Trigger frame 3210, thereby allowing the STAs time for preparing toconfigure a data frame according to their scheduling.

More specifically, each of STAs indicated by the UL MU Trigger frame3210 send the Sync signal 3221, 3222, and 3223 to an indicated orspecified resource region. Here, the Sync signal 3221, 3222, and 3223sent from each STA may be multiplexed by TDM (time divisionmultiplexing), CDM (code division multiplexing) and/or SDM (spatialdivision multiplexing).

For example, if the order of STAs indicated by the UL MU Trigger frame3210 is STA 1, STA 2, and STA 3, and the Sync signal 3221, 3222, and3223 of each STA is multiplexed by CDM, STA 1, STA 2, and STA 3 maysequentially transmit Sequence 1, Sequence 2, and Sequence 3,respectively, to the AP.

In order for each STA to multiplex the Sync signal 3221, 3222, and 3223by TDM, CDM and/or SDM and transmit them, resources (e.g.,time/sequence/streams) to be used by each STA may be indicated ordefined in advance to each STA.

Also, a PPDU for delivering the Sync signal 3221, 3222, and 3223 may notinclude an L-part, or may be transmitted by a physical layer signalalone without the MAC frame.

Having received the Sync signal 3221, 3222, and 3223 from each STA, theAP sends an Adjustment frame 3230 to each STA.

In this case, the AP may transmit the Adjustment frame 3230 to each STAby a DL SU transmission scheme or a DL MU transmission scheme. That is,for DL SU transmission, the adjustment frame 3230 may be sequentiallytransmitted to each STA participating in UL MU transmission, and for DLMU transmission the adjustment frame 3230 may be simultaneouslytransmitted to each STA participating in UL MU transmission throughresources (i.e., frequencies or streams) allocated to each STA.

The Adjustment frame 3230 may contain STA ID or address information, butthe STA ID or address information may be omitted if the order of STAsindicated in the UL MU Trigger frame 3210 also applies to the UL MUTrigger frame 3210.

Moreover, the Adjustment frame 3230 may include an Adjustment field.

The Adjustment field may contain information for adjusting differencesin timing/frequency/power. Here, adjustment information refers toinformation for correcting gaps in timing/frequency/power which may begenerated from signals the AP receives from the STAs. Besides, anyinformation may be contained in the Adjustment frame 3230 as long as itcan adjust differences in timing/frequency/power between the STAs basedon the Sync signals 3221, 3222, and 3223 received by the AP.

The PPDU for delivering the Adjustment frame 3230 may have a newstructure, even without an L-part.

Meanwhile, a procedure for sending and receiving the Sync signal 3221,3222, and 3223 and the Adjustment frame 3230 may be performed beforeeach STA transmits the UL MU Trigger frame 3210.

Moreover, the transmission of the Sync signal 3221, 3222, and 3223 maybe omitted, and the AP may include adjustment information in the UL MUTrigger frame 3210 and transmit it by implicit measurement. For example,in a pre-procedure to be described later, the AP may generate adjustmentinformation for adjusting differences in timing/frequency/power amongthe STAs through an NDP or buffer status/sounding frame which is sentfrom each STA, and send the adjustment information to each STA throughthe UL MU Trigger frame.

In addition, in the case of STAs that require no adjustment (forexample, in a case where an adjustment procedure among STAs that willperform UL MU transmission has been already completed), the procedurefor sending and receiving the Sync signal 3221, 3222, and 3223 and theAdjustment frame 3230 may be omitted.

Further, in a case where only some part requires adjustment, adjustmentmay be performed on that part only. For example, if the length of the CP(cyclic prefix) of an UL MU data frame is long enough such thatasynchrony between STAs will not be a problem, the procedure foradjusting time differences may be omitted. Also, if there is asufficiently long guard band among STAs for UL MU OFDMA transmission,the procedure for adjusting frequency differences may be omitted.

Each STA sends an UL MU Data frame 3241, 3242, and 3243 to the AP basedon the UL MU Trigger frame 3210 and Adjustment frame 3230 transmitted bythe AP. Here, each STA may receive the Adjustment frame 3230 from the APand, after an SIFS, send the UL MU Data frame 3241, 3242, and 3243 tothe AP.

The AP may send AP an ACK Frame 3250 (or BA (Block Ack) frame) inresponse to the uplink data frame 3241, 3242, and 3243 received fromeach STA. Here, the AP may receive the uplink data frame 3241, 3242, and3243 from each STA and then, after an SIFS, transmit the ACK frame 3250to each STA.

FIG. 33 is a diagram illustrating a unit of resource allocation in anOFDMA multi-user transmission technique according to an embodiment ofthe present invention.

When the DL/UL OFDMA transmission technique is used, in a PPDUbandwidth, a plurality of resource units may be defined in a unit of ntones (or subcarriers).

The resource unit means a unit of allocating the frequency resource forthe DL/UL OFDMA transmission.

One or more resource units are allocated to a single STA as a DL/ULfrequency resource, and different resource units may be allocated toeach of a plurality of STAs.

FIG. 33 illustrates the case that a PPDU bandwidth is 20 MHz.

As shown in FIG. 33, the number of tones that configure a resource unitmay be various.

For example, according to the resource unit configuration scheme shownin FIG. 33(a), a single resource unit may include 26 tones. In addition,according to the resource unit configuration scheme shown in FIG. 33(b),a single resource unit may include 52 tones or 26 tones. Further,according to the resource unit configuration scheme shown in FIG. 33(c),a single resource unit may include 106 tones or 26 tones. In addition,according to the resource unit configuration scheme shown in FIG. 33(d),a single resource unit may include 242 tones.

In the case that a resource unit is configured as shown in FIG. 33(a),up to 9 STAs may be supported in 20 MHz band for the DL/UL OFDMAtransmission. In addition, in the case that a resource unit isconfigured as shown in FIG. 33(b), up to 5 STAs may be supported in 20MHz band for the DL/UL OFDMA transmission. Further, in the case that aresource unit is configured as shown in FIG. 33(c), up to 3 STAs may besupported in 20 MHz band for the DL/UL OFDMA transmission. In addition,in the case that a resource unit is configured as shown in FIG. 32(d),the 20 MHz band may be allocated to a single STA.

Based on the number of STA that participates in the DL/UL OFDMAtransmission and/or the amount of data that the corresponding STAtransmits or receives, a resource unit configuration scheme may bedetermined among FIG. 33(a) to FIG. 33(d).

Among the whole resource units determined according to the resource unitconfiguration scheme as shown in FIG. 33(a) to FIG. 33(c), only a partof the resource units may be used for the DL/UL OFDMA transmission. Forexample, in the case that a resource unit is configured as shown in FIG.33(a) in 20 MHz, each of the resource units may be allocated to the STAsless than 9, respectively, and the remaining resource unit may not beallocated to any STA.

In the case of a DL OFDMA transmission, a data field of a PPDU istransmitted with being multiplexed in the frequency domain in a unit ofthe resource unit allocated to each STA.

On the contrary, in the case of a UL OFDMA transmission, a data field ofa PPDU may be configured in a unit of the resource unit allocated toeach STA and transmitted to an AP simultaneously. As such, since eachSTA simultaneously transmits a PPDU, an AP, which is a receiverterminal, may recognize that a data field of a PPDU transmitted fromeach STA is multiplexed and transmitted in the frequency domain.

In addition, in the case that a DL/UL OFDMA transmission and a DL/ULMU-MIMO transmission are supported at the same time, a resource unit mayinclude a plurality of streams in the spatial domain. And, one or morestreams are allocated to a single STA as a DL/UL spatial resource, anddifferent streams may be allocated to a plurality of STAs.

For example, the resource unit including 106 tones in FIG. 33(c) or theresource unit including 242 tones in FIG. 33(d) may include a pluralityof streams in the spatial domain and may support the DL/UL OFDMA and theDL/UL MU-MIMO at the same time.

The resource unit configuration scheme of the 20 MHz band in a unit of20 MHz band described above may be identically applied to the resourceunit configuration scheme of the bandwidth of 40 MHz or more. Inaddition, the smallest resource unit (i.e., a resource unit including 26tones) may be further configured in the center of the bandwidthadditionally.

FIG. 34 is a diagram illustrating a multi-user transmission procedureaccording to an embodiment of the present invention. In relation to thedrawing, the overlapped description with FIG. 31 and FIG. 32 will beomitted.

An AP may transmit a DL MU frame to at least one STA, and the STA thatreceives the DL MU frame may transmit an ACK frame (or Block Ack (BA)frame) to the AP in response to the received DL MU frame. In addition,by transmitting a UL MU trigger frame, the AP may instruct STAs that aregoing to transmit the UL MU data to prepare the UL MU transmission andprovide the information (i.e., trigger information) for an MUtransmission.

In this case, as shown in FIG. 34(a), the AP may transmit the DL MUframe and the trigger frame using different time resources (or atdifferent times). Since this is overlapped with the description inrelation to FIGS. 31 and 32, the overlapped description will be omitted.

Or, as shown in FIG. 34(b), the AP may transmit the DL MU frame and thetrigger frame using the same time resources (or at the same time). Inthis case, the DL MU frame and the trigger frame may be performedthrough the DL MU transmission at the same time with being included inthe same DL MU PPDU. As such, the scheme in which the DL MU frame andthe trigger frame are transmitted with being included in a single DL MUPPDU may be referred to as ‘cascade scheme’. The STA that receives theDL MU frame and the trigger frame together may perform the UL MUtransmission of the UL MU frame in response to the trigger frame and theACK frame in response to the DL MU frame using a single UL MU PPDU. Thatis, in response to the received DL MU PPDU, the STA may perform the ULMU transmission of a single UL MU PPDU that includes the UL MU frame andthe ACK frame.

According to the embodiment described above, since the overhead isdecreased due to the trigger frame, the SIFS and the physical preamble,an effect is occurred that the time resource is saved and the datatransmission rate increases.

Hereinafter, based on the description above, various embodiments of thepresent invention that perform the DL MU transmission of the DL MU frameand the trigger frame using a single DL MU PPDU will be described indetail.

FIG. 35 is a diagram exemplifying a PPDU structure that carries triggerinformation according to an embodiment of the present invention. It isassumed that a PPDU does not carry a DL MU frame in this embodiment, forthe convenience of description.

The trigger information for a UL MU transmission may be performedthrough a DL MU transmission with being included in a DL MU PPDU,largely, in two methods below.

1. The Case that the Trigger Information is Transmitted in NDP Format

As a first embodiment, as shown in FIG. 35(a), the trigger informationmay be performed through the DL MU transmission with being included inthe HE-SIG B field (or HE-SIG field) which is included in the physicalpreamble of a DL MU PPDU. As such, when the trigger information isincluded in the HE-SIG B field, the length of the HE-SIG B field isexcessively elongated, and thus, the communication performance may bedegraded. Accordingly, in this specification, a second HE-SIG B fieldfor the UL MU transmission may be additionally defined in addition tothe first HE-SIG field for the DL MU transmission. This will bedescribed in detail below in relation to FIGS. 36 to 38.

2. The Case that the Trigger Information is Transmitted in MAC FrameFormat

As a second embodiment, as shown in FIG. 35(b), the trigger informationmay be performed through the DL MU transmission with being included inthe MAC frame of the HE format which is included in the data field of aDL MU PPDU. In addition, although it is not shown in the drawing, thetrigger information may be performed through the DL MU transmission withbeing included in the MAC frame of the legacy format. Accordingly, themeaning of the MAC frame described below may commonly include the MACframe of the HE format and the legacy format. In this case, the MACframe including the trigger information may be referred to as a ‘triggerframe’. The trigger frame may include a single MPDU or A-MPDU formataccording to an embodiment. This will be described in more detail belowin relation to FIGS. 39 and 40.

The first embodiment described above has an effect of small overhead.The second embodiment has flexibility in that the trigger frame isconstructed using the legacy format or the HE format.

FIG. 36 is a diagram illustrating a DL MU PPDU structure according to afirst embodiment of the present invention.

Referring to FIG. 36, a DL MU PPDU according to the first embodiment ofthe present invention may include a legacy preamble (including an L-STFfield, an L-LTF field and an L-SIG filed), an HE-SIG A field thatfollows the legacy preamble, a second HE-SIC B field (or HE-SIG B forUL) 3620 that follows the HE-SIG A field, a first HE-SIG B field (orHE-SIG-B) 3610 that follows the second HE-SIC B field, an HE preamble(including HE-STF field and HE-LTF field) that follows the first HE-SIGB field 3610 and a data field that follows the preamble. The order ofthe fields described above is just an example, but may be constructed invarious orders according to embodiments. The fields described above maybe included selectively according to embodiments.

In the first HE-SIG B field 3610, the user-specific information for eachSTA that receives a data field (or to which a data field is allocated)of a DL MU PPDU may be included. Here, the user-specific informationmeans the control information that is individually required for aspecific STA among the STAs to which a resource (including frequencyresource and space resource) is allocated by the first HE-SIG B field.The user-specific information may include at least one of indicationinformation (e.g., an AID, a Partial AID (PAID), a Group Identifier(GID), etc.) for each STA (or receiving STA) to which the DL MU resource(frequency/space resource) is allocated, space resource allocationinformation for each STA, MCS information of a data field allocated foreach STA, beamforming information indicating whether to transmitbeamforming, Space Time Block Coding (STBC) information indicatingwhether to apply a space-time block coding and error correction codeinformation.

The second HE-SIG B field 3620 may include the trigger information forthe UL MU transmission. The trigger information may include at least oneof indication information for each STA (or receiving STA) that performsthe UL MU transmission and indication information of spaceresource/frequency resource for the UL MU transmission. In this case,the identifier (ID) format included in the indication information foreach STA included in the second HE-SIG B field 3620 may be differentfrom the identifier (ID) format included in the indication informationfor each STA included in the first HE-SIG B field 3610. For example, theidentifier included in the indication information for each STA includedin the second HE-SIG B field 3620 may use a newly defined ID format, notusing the AID, PAID, GID, and the like used in the first HE-SIG B field3610.

The first HE-SIG B field 3610 and the second HE-SIG B field 3620 may bedistinguished through an indication bit included in each field. Forexample, each of the first HE-SIG B field 3610 and the second HE-SIG Bfield 3620 may include an indication bit of 1 bit size.

The case that an indication bit value in a specific HE-SIG B field isset to ‘1’ may mean that the corresponding HE-SIG B field is the secondHE-SIG B field 3620, and the following first HE-SIG B field 3610 isadditionally existed. On the contrary, the case that an indication bitvalue in a specific HE-SIG B field is set to ‘0’ may mean that thecorresponding HE-SIG B field is the first HE-SIG B field 3610, and thefollowing second HE-SIG B field 3620 may not be additionally existed. Inaddition to this, the first HE-SIG B field 3610 and the second HE-SIG Bfield 3620 may be distinguished in various ways, but not limited to theembodiment described above.

In the case that a DL MU PPDU includes the first HE-SIG B field 3610 andthe second HE-SIG B field 3620 (i.e., in the case of including aplurality of HE-SIG B fields), all types of the information of the MCSlevel applied to each of the first HE-SIG B field 3610 and the secondHE-SIG B field 3620 may be provided in an HE-SIG A field. In addition,the information of the number, function, type and/or use, etc. of theHE-SIG B field may be provided together in the HE-SIG A field.

In FIG. 36, the DL MU PPDU including the first HE-SIG B field 3610 andthe second HE-SIG B field 3620 is shown in the order from the secondHE-SIG B field 3620 to the first HE-SIG B field 3610, but not limitedthereto. And, the HE-SIG B field that has lower MCS level among twoHE-SIG B fields 3610 and 3620 may be preferentially located. Here, thehigher/lower MCS level means the MCS level that indicates the modulationscheme of which data bit number per symbol is higher/lower, or the MCSlevel that indicates higher/lower code rate in the case that themodulation scheme is the same. The lower MCS level, the more beneficialfor a robust transmission.

For example, it may be assumed the case that the first HE-SIG B field3610 has the MCS level that indicates the BPSK modulation and ½ coderate and the second HE-SIG B field 3620 has the MCS level that indicatesthe QPSK modulation and ½ code rate. In this case, since the firstHE-SIG B field 3610 has lower MCS level than the second HE-SIG B field3620, a DL MU PPDU may be constructed in the order from the first HE-SIGB field 3610 to the second HE-SIG B field 3620.

In this case, the foregoing HE-SIG B field may provide the informationon whether the following HE-SIG B field is existed, configurationinformation and MCS information. That is, in the case of the drawing(FIG. 36), the second HE-SIG B field 3620 may provide the information onwhether the first HE-SIG B field 3610 is existed, the configurationinformation and the MCS information.

A data field may be allocated to at least one STA to which resource isallocated and transmitted with being multiplexed in frequency ormultiplex in space according to the resource allocation informationincluded in the first HE-SIG B field 3610.

An AP may perform the DL MU transmission of the DL MU PPDU that includesthe fields described above to at least one STA. At least one STA thatreceives the DL MU PPDU may transmit a UL MU PPDU to the AP based on thereceived DL MU PPDU. In this case, similar to the DL MU frame and thetrigger frame that are simultaneously transmitted through a single DL MUPPDU, the UL MU frame and the ACK frame (or BA frame) may also besimultaneously transmitted to the AP through a single UL MU PPDU. Thatis, the UL MU frame and the ACK frame may also be transmitted at thesame time with being included in a single UL MU PPDU in the cascadescheme. This will be described in detail in relation to FIGS. 37 and 38.

FIG. 37 is a diagram illustrating a DL MU PPDU and a UL MU PPDUtransmitted in response to the DL MU PPDU according to an embodiment ofthe present invention. In this embodiment, the HE-SIG B field of the DLMU PPDU may be constructed in the order from the first HE-SIG B field3710 to second HE-SIG B field 3720. In addition, it will be described byassuming the case that the DL MU resource is allocated to each of STAs 1to 4 and the UL MU resource is allocated to each of STAs 5 and 6. Thatis, it will be described by assuming the case that STAs 1 to 4 thatreceive the DL MU frame and STAs 5 and 6 that receive the triggerinformation are different.

Referring to FIG. 37(a), the first HE-SIG B field 3710 may includeallocation information (or user-specific information) of the DL MUresource for STAs 1 to 4. For example, the first HE-SIG B field 3710 mayinclude at least one of indication information for STAs 1 to 4,frequency resource allocation information and space resource allocationinformation. The second HE-SIG B field 3720 may include allocationinformation (or trigger information for the UL MU transmission) of theUL MU resource for STAs 5 and 6. For example, the second HE-SIG B field3720 may include at least one of indication information for STAs 5 and6, frequency resource/space resource allocation information for the ULMU transmission. Each of the data fields allocated to STAs 1 to 4 mayinclude the DL MU frame received to each of STAs 1 to 4. The data fieldmay be performed through the DL MU transmission by being multiplexed infrequency and/or multiplexed in space.

An AP may perform the DL MU transmission of a DL MU PPDU that includesthe fields described above. STAs 1 to 6 that receive the DL MU PPDU maytransmit a UL MU PPDU which is generated based on the received DL MUPPDU.

In more particular, referring to FIG. 37(b), each of STAs 1 to 4 thatreceives the DL MU frame may transmit an ACK frame (or BA frame) inresponse to the received DL MU frame. And STAs 5 and 6 that receive thetrigger information may perform the UL MU transmission of the UL MUframe generated based on the trigger information with being included ina UL MU PPDU. In this case, since the ACK frame (or BA frame) of STA 1and the UL MU frame of STA 5 are performed through the UL MUtransmission by different STAs and the UL MU resource used intransmission is also different, the ACK frame of STA 1 may not betransmitted by being piggybacked in the UL MU frame of STA 5. Inaddition, the lengths of ACK frames transmitted for each of STAs 1 to 4may also be different. That is, in the case that the STA receiving theDL MU frame and the STA receiving the trigger information through asingle DL MU PPDU are different, it may be difficult for the receivingSTAs to perform the UL MU transmission of the ACK frame and the UL MUframe using a single UL MU PPDU.

In addition, in the case that STAs 1 to 4 do not perform the UL MUtransmission of the ACK frame (or BA frame) owing to various causes suchas the case that STAs 1 to 4 fail to receive the DL MU frame, a certaintime hole interval may occur due to the absence of ACK frame (or BAframe) transmission. In the system that adopts the contention-basedchannel access scheme, a medium may be occupied by other STA during thetime hole interval, and consequently, the UL MU transmission of STAs 5and 6 that receive the trigger information may be failed.

Accordingly, in order to prevent such a problem, the configuration ofnew DL MU PPDU and the UL MU PPDU transmitted in response to the DL MUPPDU is proposed with referent to FIG. 38 below.

FIG. 38 is a diagram illustrating a DL MU PPDU and a UL MU PPDUaccording to another embodiment of the present invention. In thisembodiment, the HE-SIG B field of the DL MU PPDU may be constructed inthe order from the first HE-SIG B field 3810 to second HE-SIG B field3820. In addition, it will be described by assuming the case that the DLMU resource is allocated to each of STAs 1 to 4 and the UL MU resourceis allocated to each of STAs 1 and 2. That is, it will be described byassuming the case that STAs 1 and 2 that receive the trigger informationis a subset of STAs 1 to 4 that receive the DL MU frame.

Referring to FIG. 38(a), the first HE-SIG B field 3810 may includeallocation information (or user-specific information) of the DL MUresource for STAs 1 to 4. For example, the first HE-SIG B field 3810 mayinclude at least one of indication information for STAs 1 to 4,frequency resource allocation information and space resource allocationinformation.

The second HE-SIG B field 3820 may include allocation information (ortrigger information for the UL MU transmission) of the DL MU resourcefor STAs 1 and 2. For example, the second HE-SIG B field 3820 mayinclude at least one of indication information for STAs 1 and 2,frequency resource/space resource allocation information for the UL MUtransmission.

The data field may include the DL MU frame received by each of STAs 1 to4 to which the DL MU resource is allocated. The data field may beperformed through the DL MU transmission by being multiplexed infrequency and/or multiplexed in space.

An AP may perform the DL MU transmission of a DL MU PPDU that includesthe fields described above. STAs 1 to 4 that receive the DL MU PPDU mayperform the UL MU transmission of a UL MU PPDU which is generated basedon the received DL MU PPDU.

In more particular, referring to FIG. 38(b), each of STAs 1 to 2 thatreceives the DL MU frame and the trigger information together mayperform the UL MU transmission by piggybacking an ACK frame (or BAframe) in response to the received DL MU frame in the UL MU framegenerated based on the received trigger information. Hereinafter, forthe convenience of description, the frame in which the ACK frame (or BAframe) is piggybacked in the UL MU frame may be referred to as ‘cascadeUL MU frame’. Such a cascade UL MU frame may be performed through the ULMU transmission by identically using the DL MU resource used whentransmitting the DL MU frame received in each of STAs 1 and 2.

In addition, STAs 3 and 4 that receive the DL MU frame may perform theUL MU transmission of the ACK frame (or BA frame) in response to thereceived DL MU frame. Even in this case, the ACK frame (or BA frame) maybe performed through the UL MU transmission by identically using the DLMU resource used when transmitting the DL MU frame received in each ofSTAs 3 and 4.

Since each STA performs the UL MU transmission using the resource whichis the same as the transmission resource of the DL MU frame receivedthrough a single DL MU PPDU, the cascade UL MU frame and the ACK frame(BA frame) described above may be performed the UL MU transmissionthrough a single and the same UL MU PPDU. In this case, each ACK frame(BA frame) may be padded so as to have the bit size which is the same asthe maximum bit size of the UL MU frame.

Referring to FIG. 38(c), the AP that receives the UL MU PPDU which istransmitted through the UL MU by STAs 1 and 2 may perform the DL SUtransmission of the ACK frame (or the BA frame) in response to thereceived UL MU frame to STAs 1 and 2. In this case, the AP may performthe DL SU transmission of the ACK frame (or the BA frame) using a DL SUPPDU that has the same bandwidth as that of the received UL MU PPDU. Inaddition, the ACK frame (or the BA frame) may be transmitted through theDL SU by being duplicated in a unit of 20 MHz within the DL MU PPDU.

In this embodiment, a certain time hole does not occur due to thetransmission failure of the ACK frame (or the BA frame) performed bySTAs 1 to 4 (since it is transmitted through the same UL MU PPDU), theproblem does not occur that the UL MU frame transmission of STAs 1 and 2is failed owing to the medium occupation of other STA.

Considering the contents described above in relation to FIG. 37 and FIG.38, in the case that the STA receiving the trigger information is asubset of the STA receiving the DL MU frame, it may be identified thatthe UL MU frame transmission of the STA is not failed. In other words,in the case that a part of the STAs receiving the DL MU frame performsthe transmission of the UL MU frame, it may be identified that thecorresponding UL MU frame transmission is not failed.

Accordingly, in order to prevent the transmission failure of such a ULMU frame, in the case that a subset condition is satisfied between theSTA receiving the trigger information and the STA receiving the DL MUframe, an AP may perform the DL MU transmission of the DL MU PPDU inwhich the trigger information and the DL MU frame are included. As aresult, in the UL MU PPDU which is performed through the UL MUtransmission that corresponds to the DL MU PPDU, at least one cascade ULMU frame may be existed.

In addition, in order to prevent the transmission failure of the UL MUframe and to use efficiently the resource (frequency, time, spaceresource), another UL MU PPDU structure may be proposed, which performsthe UL MU transmission of the ACK frame (or BA frame) and the UL MUframe using a single UL MU PPDU. In this case, the subset condition maynot be satisfied between the STA of the DL MU reception and the STA ofthe UL MU transmission. This will be described in detail in relation toFIG. 42.

Meanwhile, the ACK frame (or the BA frame) may be applied to the ACKframe (or the BA frame) format defined in the 802.11a system or the802.11ax system. However, in the case that the ACK frame (or the BAframe) is transmitted together with the UL MU frame, the ACK frame (orBA frame) may be applied to the ACK frame (or BA frame) format definedin the 802.11ax system.

Hereinafter, for the convenience of description, the ACK frame and theBA frame are commonly called ‘ACK frame’.

In the embodiment described above, the embodiment of performing the ULMU transmission using the same resource as the transmission resource ofthe DL MU frame which is received through a single DL MU PPDU has beendescribed. However, the cascade UL MU frame and/or the UL MU frame maybe transmitted through the UL MU using a resource that is different fromthe DL MU transmission resource of the DL MU frame. In this case, theHE-SIG B field may separately provide a frame format that an STA isgoing to perform the UL MU transmission and the indication informationsuch as the UL MU transmission resource allocated to each STA. This willbe described in detail below.

1) First Option

A method may be proposed that a UL MU resource is preferentiallyallocated to the STA (or the STA that is going to perform the UL MUtransmission of the ACK frame only) which is included in a first HE-SIGB field 3810 but not included in a second HE-SIG B field 3820, and theremaining resource is allocated for a resource that is going to transmitthe cascade UL MU frame and/or the UL MU frame. In this case, the typeand size of the UL MU resource that is preferentially allocated to eachSTA frame may be preconfigured, and the UL MU resource may be allocated,which is preconfigured in an order of STA ID for receiving the DL MUframe included in the first HE-SIG B field 3810.

For example, the case may be assumed that STAs 1 to 4 are sequentiallyincluded in the first HE-SIG B field 3810 and the STAs that are going toperform the UL MU transmission of the ACK frame only are STAs 2 and 3.In addition, it may be assumed that the UL MU resource allocated for theACK frame transmission is preconfigured as a single 26-tone resourceunit. In this case, as the resource for transmitting the ACK frame, theresource may be allocated one by one in the order from the first 26-toneresource unit to STA 2, and to STA 3. The remaining resource unit afterbeing allocated to the STA may be allocated to a receiving STA IDincluded in the second HE-SIG B field 3820.

2) Second Option

A method may be proposed that the ACK frame format for each receivingSTA of the DL MU frame and the transmission resource are indicated inthe first HE-SIG B field 3810. For example, the first HE-SIG B field3810 may include an ACK frame indicator for indicating the ACK frameformat for each receiving STA and the transmission resource. The ACKframe indicator may indicate whether the corresponding STA is the STAthat is needed to transmit the ACK frame or the STA that is needed totransmit the cascade UL MU frame (ACK frame+UL MU frame). In the casethat the corresponding STA is the STA that is needed to transmit the ACKframe, the ACK frame indicator may additionally indicate the informationof the UL MU transmission resource that is going to be used fortransmitting the ACK frame.

For example, the case may be assumed that the bit size of the ACK frameindicator is 1 bit.

In this case, when the bit value of the ACK frame indicator is set to‘0’, the corresponding ACK frame indicator may indicate that thecorresponding STA is the STA that is needed to transmit the cascade ULMU frame or the STA (in the case that the DL MU frame is not the ACKframe) that is not needed to transmit the ACK frame.

On the contrary, when the bit value of the ACK frame indicator is set to‘1’, the corresponding ACK frame indicator may indicate that thecorresponding STA is the STA that is needed to transmit the ACK frame.

The STA that is needed to transmit the ACK frame may transmit the ACKframe using the UL MU resource of a preconfigured size, which isallocated in the order of the receiving STA ID included in the firstHE-SIG B field 3810, which is the same with the first option. Or, asdescribed above, the ACK frame indicator may additionally indicate theinformation of the UL MU transmission resource that is needed to be usedfor transmitting the ACK frame. In this case, the STA may perform the ULMU transmission of the ACK frame using the UL MU resource indicated bythe ACK frame indicator.

3) Third Option

A method may be proposed that a format of the frame that is going toperform the UL MU transmission and the information of the UL MUtransmission resource of the corresponding frame may be provided foreach STA in the second HE-SIG B field 3820. The second HE-SIG B field3820 may indicate whether to transmit the ACK frame, the cascade UL MUframe or the UL MU frame as the frame format for each receiving STA (orthe receiving STA that receives the DL MU frame) included in the firstHE-SIG B field 3810. In this case, the second HE-SIG B field 3820 mayindicate the frame format that corresponds to each receiving STA byreference to the receiving STA ID included in the first HE-SIG B field3810 (e.g., in the order of the receiving STA ID included in the firstHE-SIG B field 3810). Further, the second HE-SIG B field 3820 may alsoprovide the allocation information of the UL MU resource that is goingto be used for performing the UL MU transmission of each frame (the ACKframe, the cascade UL MU frame or the UL MU frame) for each STA.

4) Fourth Option

A method may be proposed that the UL MU resource is preferentiallyallocated to the STAs that transmit the cascade UL MU frame or the UL MUframe, and the remaining UL MU resource is allocated to the STA thattransmits the ACK frame. Such resource allocation information for eachSTA may be provided through the second HE-SIG B field 3820.

5) Fifth Option

A method may be proposed that the receiving STA receiving the DL MUframe together with the trigger information is indicated in bitmapformat in the second HE-SIG B field 3820. For example, when the numberof STAs that receive the trigger information of the second HE-SIG Bfield 3820 is N, after N bits is configured in the second HE-SIG B field3820, the bit value of the location that corresponds to the STA thatadditionally receives the DL MU frame within N bits may be set to ‘1’.In this case, the second HE-SIG B field 3820 may also be additionallyindicated the UL MU resource allocation information for transmitting thecascade UL MU frame to the STAs corresponding to the bit position havingthe value ‘1’. On the contrary, the UL MU resource allocationinformation for transmitting the ACK frame may be indicated to the STAscorresponding to the bit position having the value ‘0’ in the methodproposed in the first to fourth options.

In addition, the second HE-SIG B field 3820 may indicate the UL MUresource allocated to the STA ID of the corresponding STAs and each STAin the case that the STAs transmitting the UL MU frame only are existed.

So far, the embodiment that the trigger information is transmitted withbeing included in the physical preamble (i.e., the HE-SIG B field of thephysical preamble) of the DL MU PPDU has been described in detail.Hereinafter, the embodiment that the trigger frame (i.e., the MAC frameformat of the trigger information) including the trigger information istransmitted with being included in a data field of the DL MU PPDU willbe described in detail.

In the case that the trigger frame may be constructed in the MAC framestructure, the trigger frame may include a single MPDU. In this case,the trigger frame may correspond to a type of the MAC control frame orthe MAC management frame, and a trigger frame type may be newly definedin different way from it.

Or, the trigger frame may include the frame (e.g., HT-control wrapperframe) that includes the HT-control field in the MAC header.

In this case, the information for performing the UL MU transmission(i.e., the trigger information) may be included in the HT-control fieldin the corresponding frame. Or, the trigger information may be includedin the newly defined HE-control field as an HE format in the 802.11axsystem. The HE-control field may be newly defined in the 802.11ax systemin the similar way of the HT-control field defined in the conventionalHT format. Accordingly, the HE-control field may be included in the MACheader in the similar way of the method that the HT-control fieldexemplified in FIG. 6 above is included in the MAC header.

The trigger frame may be divided into two structures, largely, dependingon the way of being included in a DL MU PPDU as follows.

1) Embodiment 2-1

In the case that a part of the STAs that receive the DL MU frameperforms the UL MU transmission (i.e., the UL MU transmitting STA is asubset of the DL MAC frame receiving STA), a DL MU PPDU may include anA-MPDU in a data field, and the corresponding A-MPDU may include atleast one MPDU that corresponds to the trigger frame and the remainingMPDU that corresponds to the DL MU frame. In this case, the informationfor the UL MU transmission (i.e., the trigger information) may beincluded in the MAC header in the corresponding A-MPDU or in the MACframe body. This will be described in more detail in relation to FIG. 39below.

2) Embodiment 2-2

Or, in the case that the STA receiving the DL MU frame does not performthe UL MU transmission (i.e., the UL MU transmitting STA is not a subsetof the DL MAC frame receiving STA), a DL MU PPDU may include the MPDUonly that corresponds to the trigger frame in a data field. In thiscase, the information for the UL MU transmission (i.e., the triggerinformation) may be included in the MAC header in the correspondingA-MPDU or in the MAC frame body. This will be described in more detailin relation to FIG. 40 below.

FIG. 39 is a diagram illustrating a DL MU PPDU structure according tothe embodiment 2-1 of the present invention.

Referring to FIG. 39, in the case that a part of the STAs that receivethe DL MU frame performs the UL MU transmission (or, in the case thatthe STA receiving the DL MU frame and the trigger frame together isexisted), the data field allocated to each of the part of the STAs maybe constructed by the A-MPDU format as the example of FIG. 17. In thiscase, at least one MPDU among a plurality of MPDUs that constructs theA-MPDU may correspond to the trigger frame, and the remaining MPDU maycorrespond to the DL MU frame.

In this case, the position of the MPDU that includes the triggerinformation may be pre-designated. For example, the first MPDU among aplurality of MPDUs that constructs the A-MPDU may correspond to thetrigger frame, and the remaining MPDU may correspond to the DL MU frame.In this case, an AP may indicate the pre-designated position of the MPDUto the STAs through a beacon frame and so on.

And, the data field allocated to the remaining STAs except the part ofSTAs may be constructed by the MPDU or the A-MPDU format, and maycorrespond to the DL MU frame.

Since the trigger frame included in a single A-MPDU and the receivingSTA of the DL MU frame are the same, the receiving STA information ofthe trigger frame may not be separately requested. Accordingly, thetrigger frame may not separately include the information of the STA thatperforms the UL MU transmission. The MPDU that corresponds to thetrigger frame may include an indicator for indicating that thecorresponding MPDU includes the trigger information. For example, theMPDU that includes the trigger information may indicate that thecorresponding MPDU corresponds to the trigger frame using the framesubtype included in the MAC header.

Since the embodiment 2-1 described above is applied to the case that theUL MU transmitting STA is a subset of the DL MU frame receiving STA, apredetermined restriction may be existed in the UL MU scheduling.

Meanwhile, in this embodiment, the trigger frame transmitted to the STAthat participates in the DL MU transmission may be referred to as a‘unicast trigger frame’.

FIG. 40 is a diagram illustrating a DL MU PPDU structure according tothe embodiment 2-2 of the present invention.

Referring to FIG. 40, in the case that the STA that receives the DL MUframe does not perform the UL MU transmission (or, in the case that theSTA receiving the DL MU frame and the trigger frame together is notexisted), the trigger frame may be included in a specific data fieldamong the data fields of the DL MU PPDU that carries the DL MU frame.More particularly, the transmission resource of the data field of the DLMU PPDU that carries the DL MU frame may be multiplexed according to theDL OFDMA and/or the DL MU-MIMO scheme, and the trigger frame may becarried using a specific data field of the multiplexed data fields. Forexample, the trigger frame may be included in the data field thatcorresponds to a sub band that is predetermined for transmitting thetrigger frame.

In the case that the trigger frame is included in a specific data fieldamong the data fields of the DL MU PPDU that carries the DL MU frame,the corresponding specific data field may include only a single MPDUthat corresponds to the trigger frame. In this case, the information forthe UL MU transmission (i.e., the trigger information) may be includedin the MAC header or the MAC frame body in the corresponding MPDU.Particularly, in this embodiment, since the UL MU transmitting STA andthe DL MU receiving STA are different, the STA information for receivingthe trigger information may be additionally requested in addition to theDL MU receiving STA information included in the HE-SIG B field.Accordingly, different from the embodiment 2-1, in the embodiment 2-2,both of the STA information receiving the trigger frame (or the STAinformation that is going to perform the UL MU transmission) as thetrigger information and the UL MU resource allocation informationallocated to each STA may be included in the MPDU that corresponds tothe trigger frame.

The specific data field that includes the trigger frame may betransmitted in broadcast way. Accordingly, the AID of the specific datafield may be set to a broadcast AID, and the AID of the STAs to whichthe UL MU transmission is triggered may be included in the trigger frameof the specific data field. Accordingly, a single MPDU included in thespecific data field (or the trigger frame included in the correspondingMPDU) may be referred to as a ‘broadcast trigger frame’. In other words,the trigger frame transmitted to the STA that does not participate inthe DL MU transmission may be referred to as a ‘broadcast triggerframe’.

In the case that only one MPDU corresponding to the trigger frame isincluded in the trigger frame of the specific data field of the DL MUPPDU, it is required to notify that the specific data field includingthe trigger frame is existed to an STA.

In this case, in the HE-SIG B filed that corresponds to thecorresponding data field, a trigger indicator indicating that thetrigger frame is included in the corresponding data field may beincluded. For example, the trigger indicator of 1 bit size thatindicates whether a specific data field including the triggerinformation is included may be included in the HE-SIG B filed thatprovides user-specific information of the specific data field.

Or, as described above, the HE-SIG B filed may include the broadcast AIDas indication information of the corresponding data filed for thebroadcast transmission of the specific data field. Accordingly, the STAmay recognize that the specific data filed including the trigger frameis existed through the broadcast identification information included inthe HE-SIG B filed.

In the case that the trigger frame is included in a specific data fieldamong the data fields of the DL MU PPDU that carries the DL MU frame,the position (e.g., position of sub band, DL MU STA index, etc.) of thedata field that includes the trigger frame may be pre-designated. Inthis case, an AP may notify the position of the pre-designated specificdata field to STAs through a beacon frame, and the like.

The bit size of a specific data field that includes a single MPDU may besmaller than that of other data field (e.g., data field includingA-MPDU). Accordingly, the specific data field is padded and included ina DL MU PPDU so as to have the same bit size as other data field thathas the maximum bit size (or so as to be the same as the length of a DLMU PPDU).

Since the embodiment 2-2 may be applied to the case that a UL MUtransmitting STA is not a subset of a DL MU frame receiving STA, the ULMU scheduling has more beneficial effect.

So far, the DL MU PPDU structure according to the second embodiment ofthe present invention has been described. The embodiments 2-1 and 2-2described above may be independently applied or applied in combinationaccording to a situation. In the case that the embodiments 2-1 and 2-2are applied in combination, a specific data field of a DL MU PPDU mayinclude only one MPDU that corresponds to the trigger frame, and otherdata field may include the trigger frame and the DL MU frame as theA-MPDU format.

For example, in the case that the DL MU receiving STA is STAs 1 and 2,and the UL MU transmitting STA is STAs 2 and 3, the trigger frame forSTA 3 may be included in the first data field as an MPDU, the triggerframe and the DL MU frame for STA 2 may be included in the second datafield as the A-MPDU format, and the DL MU frame for STA 1 may beincluded in the third data field as the MPDU format or the A-MPDUformat.

FIG. 41 illustrates a table comparing the first and second embodiments.

Referring to FIG. 41, the first embodiment has the following advantageand effect.

-   -   Flexibility of UL MU scheduling.    -   Advantageous in the aspect of Resource saving/Power saving of        other STAs/UL MU STAs.

However, in the first embodiment, a problem may be existed that a bitfor indicating the ACK frame is additionally requested in the HE-SIG Bfield (refer to the second option). In addition, in the firstembodiment, a disadvantage may be existed that a new HE-SIG B field(i.e., second HE-SIG B field) is requested for indicating the UL MUresource allocation information.

The embodiment 2-1 has the following advantage and effect.

-   -   Advantageous in the aspect of Resource saving/Power saving of        other STAs/UL MU STAs.    -   Available to use the conventional DL MU frame format as it is.

However, since the embodiment 2-1 is an embodiment that may be appliedin the case that the UL MU transmitting STA is a subset of the DL MUframe receiving STA, a problem is existed that a predeterminedrestriction is existed in the UL MU scheduling.

The embodiment 2-2 has the following advantage and effect.

-   -   Free to make a UL MU group or Flexibility of UL MU scheduling.

However, in the embodiment 2-2, a problem may be existed that a bit forindicating whether the trigger frame is existed is additionallyrequested in the HE-SIG B field. In addition, a problem may be existedin that the resource and power of the UL MU transmitting STAs and otherSTAs may be wasted. This is because a specific data field including theMPDU only that corresponds to the trigger frame is transmitted withbeing padded so as to have the same length as other data field.Accordingly, the DL MU resource is wasted, and a problem occurs that theSTAs according to the 802.11ax system may not be slept (power waste)during the DL MU PPDU length in order to decode the trigger frame.

Hereinafter, a UL MU PPDU structure will be described, which isperformed through the UL MU transmission that corresponds to the DL MUPPDU of the first and second embodiments (including embodiments 2-1 and2-2) described above.

FIG. 42 is a diagram illustrating a UL MU PPDU according to anembodiment of the present invention. In this embodiment, the case isassumed and described that the DL MU frame is received in STAs 1 and 2,and the trigger frame is received in STAs 2 and 3.

The STA that receives the DL MU PPDU may generate a UL MU PPDU based onthe received DL MU PPDU and perform the UL MU transmission to an AP. Inthis case, each STA may transmit the UL MU frame based on the triggerframe included in the DL MU PPDU, and may transmit an ACK frame inresponse to the DL MU frame. In this case, the method of performing theUL MU transmission of the UL MU frame and the ACK frame may be differentaccording to embodiments.

Referring to FIG. 42(a), as an embodiment, each of the UL MU frame andthe ACK frame may be performed through the UL MU transmission usingseparate UL MU resources. For example, the frequency resource may bedivided into two regions, largely, and a first region thereof may beused for the frequency resource for the ACK frame transmission, and asecond region thereof may be used for the frequency resource for the ULMU frame transmission. Further, the first region may be allocated withbeing separated to the STAs that transmit the ACK frame again, and thesecond region may also be allocated with being separated to the STAsthat transmit the UL MU frame.

Accordingly, the first region for the ACK frame transmission may beallocated to STA 1 and STA 2, respectively, and the second region forthe UL MU frame transmission may be allocated to STA 2 and STA 3,respectively. As a result, each of the ACK frame of STA 1, the ACK frameof STA 2, the UL MU frame of STA 2 and the UL MU frame of STA 3 may beperformed through the UL MU transmission using different UL MU resourcesthrough a single UL MU PPDU.

The present embodiment has an advantage that the UL MU transmissionscheme of each frame is simple. However, since the STA (STA 2, in theabove embodiment) that may transmit the ACK frame and the UL MU frame atthe same time transmits each frame using separate UL MU resources, aproblem is in that the UL MU resource is wasted. In addition, a problemis existed in that the resource is wasted on the point that the APreceiving the corresponding UL MU PPDU is needed to decode two OFDMApackets (the OFDMA packet including the ACK frame of STA 2 and the OFDMApacket including the UL MU frame of STA 2) separately. In order to solvesuch a problem, the following UL MU transmission method may be proposed.

As another embodiment, referring to FIG. 42(b), the ACK frametransmitted by the same STA may be transmitted with being piggybacked inthe UL MU frame. For example, STA 2 that receives the DL MU frame andthe trigger frame together may perform the UL MU transmission of the ACKframe in response to the corresponding DL MU frame with beingpiggybacked in the UL MU frame for the corresponding trigger frame. Inthis case, the data field allocated to STA 2 for the UL MU transmissionmay be constructed in the A-MPDU format. And, at least one of aplurality of MPDUs that construct the A-MPDU may correspond to the ACKframe, and the remaining MPDU may correspond to the UL MU frame.

In the present embodiment, the UL MU resource allocation information fortransmitting the ACK frame of STA 2 is not separately requested. This isbecause STA 2 only needs to transmit the UL MU frame with piggybackingthe ACK frame. Accordingly, when performing the DL MU transmission, anAP may not separately include the UL MU resource allocation informationfor the ACK frame of STA 2 in the HE-SIG B field of the DL MU PPDU. Inaddition, the MCS level of the UL MU frame may be applied to the MCSlevel applied to the ACK frame.

Meanwhile, the embodiment described in FIGS. 37 and 38 may be applied to38 the allocation method of the UL MU resource in the same way, and theoverlapped description will be omitted.

The present embodiment has an effect that the resource is saved.However, the UL MU transmission method is more complex that thetransmission method described above, and a problem is existed that theMCS level proper to a robust transmission (or lower MCS level) may notbe applied to the piggybacked ACK frame (since the MCS level which isthe same as the UL MU frame may be applied in the same data field).

FIG. 43 is a flowchart illustrating a DL MU transmission methodperformed by an AP according to an embodiment of the present invention.The embodiments described above may be identically applied to thedescription in relation to the flowchart. Accordingly, the overlappeddescription will be omitted below.

Referring to FIG. 43, an AP may transmit a DL MU PPDU (step, S4310).More particularly, the AP may perform the DL MU transmission of the DLMU PPDU that includes a physical preamble and a data field. In thiscase, the DL MU PPDU may be a field that includes the trigger frame andthe DL MU frame that includes the trigger information for triggering theUL MU transmission.

Next, the AP may perform the UL MU reception of the UL MU PPDU which isgenerated based on the DL MU PPDU (step, S4320). In this case, the UL MUPPDU may include the ACK frame which is in response to the UL MU frameand the DL MU frame based on the trigger frame.

FIG. 44 is a block diagram of each STA device according to an embodimentof the present invention.

In FIG. 44, an STA device 4410 may include a memory 4412, a processor4411 and an RF unit 4413. And, as described above, the STA device may bean AP or a non-AP STA as an HE STA device.

The RF unit 4413 may transmit/receive a radio signal with beingconnected to the processor 4411. The RF unit 4413 may transmit a signalby up-converting the data received from the processor 4411 to thetransmission/reception band.

The processor 4411 may implement the physical layer and/or the MAC layeraccording to the IEEE 802.11 system with being connected to the RF unit4013. The processor 4411 may be constructed to perform the operationaccording to the various embodiments of the present invention accordingto the drawings and description. In addition, the module forimplementing the operation of the STA 4410 according to the variousembodiments of the present invention described above may be stored inthe memory 4412 and executed by the processor 4411.

The memory 4412 is connected to the processor 4411, and stores varioustypes of information for executing the processor 4411. The memory 4412may be included interior of the processor 4411 or installed exterior ofthe processor 4411, and may be connected with the processor 4411 by awell known means.

In addition, the STA device 4410 may include a single antenna or amultiple antenna.

The detailed construction of the STA device 4410 of FIG. 44 may beimplemented such that the description of the various embodiments of thepresent invention is independently applied or two or more embodimentsare simultaneously applied.

The embodiments described above are constructed by combining elementsand features of the present invention in a predetermined form. Theelements or features may be considered optional unless explicitlymentioned otherwise. Each of the elements or features can be implementedwithout being combined with other elements. In addition, some elementsand/or features may be combined to configure an embodiment of thepresent invention. The sequential order of the operations discussed inthe embodiments of the present invention may be changed. Some elementsor features of one embodiment may also be included in anotherembodiment, or may be replaced by corresponding elements or features ofanother embodiment. Also, it will be obvious to those skilled in the artthat claims that are not explicitly cited in the appended claims may bepresented in combination as an exemplary embodiment of the presentinvention or included as a new claim by subsequent amendment after theapplication is filed.

The embodiments of the present invention may be implemented throughvarious means, for example, hardware, firmware, software, or acombination thereof. When implemented as hardware, one embodiment of thepresent invention may be carried out as one or more application specificintegrated circuits (ASICs), one or more digital signal processors(DSPs), one or more digital signal processing devices (DSPDs), one ormore programmable logic devices (PLDs), one or more field programmablegate arrays (FPGAs), a processor, a controller, a microcontroller, amicroprocessor, etc.

When implemented as firmware or software, one embodiment of the presentinvention may be carried out as a module, a procedure, or a functionthat performs the functions or operations described above. Software codemay be stored in the memory and executed by the processor. The memory islocated inside or outside the processor and may transmit and receivedata to and from the processor via various known means.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent invention. The above exemplary embodiments are therefore to beconstrued in all aspects as illustrative and not restrictive. The scopeof the invention should be determined by the appended claims and theirlegal equivalents, not by the above description, and all changes comingwithin the meaning and equivalency range of the appended claims areintended to be embraced therein.

While a frame transmission scheme in a wireless communication systemaccording to the present invention has been described with respect toits application to an IEEE 802.11 system, it also may be applied toother various wireless communication systems than the IEE 802.11 system.

What is claimed is:
 1. A Station (STA) in a wireless communicationsystem, the STA comprising: a transceiver configured to transmit andreceive a wireless signal; and a processor configured to control thetransceiver, wherein the processor is further configured to: receive,from an access point (AP), a downlink (DL) multi-user (MU) PhysicalProtocol Data Unit (PPDU), wherein the DL MU PPDU includes a DL data andtrigger frame for an uplink (UL) orthogonal frequency-division multipleaccess (OFDMA) transmission; and transmit, to the AP, an UL MU PPDUgenerated based on the DL MU PPDU, wherein the trigger frame istransmitted in a first frequency region of the DL MU PPDU, and the DLdata is transmitted in a second frequency region of the DL MU PPDU, whenthe trigger frame is for multiple stations (STAs), and wherein the firstfrequency region and the second frequency region are different frequencyregion.
 2. The STA of claim 1, wherein the trigger frame and the DL dataare transmitted in a same frequency region of the DL MU PPDU for thesingle STA, and an acknowledge (ACK) frame in response to the DL dataand an UL MU frame in response to the trigger frame are transmitted in asame frequency region in the UL MU PPDU, when the trigger frame is for asingle STA.
 3. The STA of claim 1, wherein a High Efficiency-Signal(HE-SIG) B field in the DL MU PPDU includes Broadcast AssociationIdentifier (AID) information for the trigger frame, when the triggerframe is for the multiple STAs.
 4. The STA of claim 1, wherein thetrigger frame includes at least one of Association Identifier (AID)information of an STA that performs the UL OFDMA transmission, spaceresource indication information for the UL OFDMA transmission orfrequency resource allocation information for the UL OFDMA transmission.5. A method for performing an uplink (UL) multi-user (MU) transmissionby a station (STA) in a wireless communication system, the methodcomprising: receiving, from an access point (AP), a downlink (DL)multi-user (MU) Physical Protocol Data Unit (PPDU), wherein the DL MUPPDU includes a DL data and trigger frame for an uplink (UL) orthogonalfrequency-division multiple access (OFDMA) transmission; andtransmitting, to the AP, an UL MU PPDU generated based on the DL MUPPDU, wherein the trigger frame is transmitted in a first frequencyregion of the DL MU PPDU, and the DL data is transmitted in a secondfrequency region of the DL MU PPDU, when the trigger frame is formultiple stations (STAs), and wherein the first frequency region and thesecond frequency region are different frequency region.
 6. The method ofclaim 5, wherein the trigger frame and the DL data are transmitted in asame frequency region of the DL MU PPDU for the single STA, and anacknowledge (ACK) frame in response to the DL data and an UL MU frame inresponse to the trigger frame are transmitted in a same frequency regionin the UL MU PPDU, when the trigger frame is for a single STA.
 7. Themethod of claim 5, wherein a High Efficiency-Signal (HE-SIG) B field inthe DL MU PPDU includes Broadcast Association Identifier (AID)information for the trigger frame, when the trigger frame is for themultiple STAs.
 8. The method of claim 5, wherein the trigger frameincludes at least one of Association Identifier (AID) information of anSTA that performs the UL OFDMA transmission, space resource indicationinformation for the UL OFDMA transmission or frequency resourceallocation information for the UL OFDMA transmission.